Once the passage is read in its intended context, it becomes quite clear that the descendants that were to inherit the promise land could not be the Ishmaelites. The promise could only be referring to the descendants of Isaac, i.e. Israel. It was the Israelites that served as slaves in a foreign land, namely Egypt, for four generations only to return to Canaan afterwards: 

MY RESPONSE:

The biblical text was not misquoted by me. The critic has ADDED three more verses to show what he calls the "intended context". There is nothing within the additional text that tells us that the God's Covenant for the Land was made subject to the serving of the Israelites as slaves in a foreign land or in Egypt.
However, the Critic overlooks the NEXT VERSE immediately after what I have quoted (Genesis 15: 18-21). This verse reads: "Now Sarai, Abram's wife, bore him no children. She had an Egyptian slave girl whose name was Hagar."
This verse explicitly tells us that his sons, Ishmael and Isaac, both were born after the above quoted "Covenant for the Land". Hence, it applies to BOTH.

"And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him. And he said unto Abram, Know of a surety that THY SEED (zera) SHALL BE A STRANGER IN A LAND THAT IS NOT THEIR’S, and shall serve them; and they shall afflict THEM four hundred years; And also that nation, whom they shall serve, will I judge: and afterward SHALL THEY COME OUT with great substance. And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age. But in the fourth generation THEY COME HITHER AGAIN: for the iniquity of the Amorites is not yet full. And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces. In the same day the LORD made a covenant with Abram, saying, Unto THY SEED (zera) have I given this land, from the river of Egypt unto the great river, the river Euphrates: The Kenites, and the Kenizzites, and the Kadmonites, And the Hittites, and the Perizzites, and the Rephaims, And the Amorites, and the Canaanites, and the Girgashites, and the Jebusites." Genesis 15:12-21

"Judah and Israel were many, as the sand which is by the sea in multitude, eating and drinking, and making merry. And Solomon reigned over all kingdoms from the river unto the land of the Philistines, and unto the border of Egypt: they brought presents, and served Solomon all the days of his life... For he had dominion over all the region on this side the river, from Tiphsah even to Azzah, over all the kings on this side the river: and he had peace on all sides round about him. And Judah and Israel dwelt safely, every man under his vine and under his fig tree, from Dan even to Beersheba, all the days of Solomon." 1 Kings 4:20-21, 24-25

"Also at the same time Solomon kept the feast seven days, and all Israel with him, a very great congregation, from the entering in of Hamath unto the river of Egypt." 2 Chronicles 7:8

"And Solomon had four thousand stalls for horses and chariots, and twelve thousand horsemen; whom he bestowed in the chariot cities, and with the king at Jerusalem. And he reigned over all the kings from the river even unto the land of the Philistines, and to the border of Egypt." 2 Chronicles 9:25-26

"But Abram said, ‘O Sovereign Lord, what can you give me since I remain childless and the one who will inherit my estate is Eliezer of Damascus?’ And Abram said, ‘You have given me no children; so a servant in my household will be my heir.’ Then the word of the Lord came to him: ‘This man will not be your heir, but a son coming from your own body will be your heir.’ He took him outside and said, "Look up at the heavens and count the stars-if indeed you can count them.’ Then he said to him, ‘So shall your offspring be.’ Abram believed the Lord, and he credited it to him as righteousness." Genesis 15:2-6

"But Sarah saw that the son whom Hagar the Egyptian had borne to Abraham was mocking, and she said to Abraham, ‘Get rid of that slave woman and her son, for that slave woman’s son will never share IN THE INHERITANCE WITH MY SON ISAAC.’ The matter distressed Abraham greatly because it concerned his son. But God said to him, ‘Do not be so distressed about the boy and your maidservant. Listen to whatever Sarah tells you, BECAUSE IT IS THROUGH ISAAC THAT YOUR OFFSPRING WILL BE RECKONED. I will make the son of the maidservant into a nation also, because he is your offspring.’" Genesis 21:9-13

"Abraham took another wife, whose name was Keturah. She bore him Zimran, Jokshan, Medan, Midian, Ishbak and Shuah. Jokshan was the father of Sheba and Dedan; the descendants of Dedan were the Asshurites, the Letushites and the Leummites. The sons of Midian were Ephah, Epher, Hanoch, Abida and Eldaah. All these were descendants of Keturah. ABRAHAM LEFT EVERYTHING HE OWNED TO ISAAC. But while he was still living, he gave gifts to the sons of his concubines and sent them away from his son Isaac to the land of the east. Altogether, Abraham lived a hundred and seventy-five years. Then Abraham breathed his last and died at a good old age, an old man and full of years; and he was gathered to his people. His sons Isaac and Ishmael buried him in the cave of Machpelah near Mamre, in the field of Ephron son of Zohar the Hittite, the field Abraham had bought from the Hittites. There Abraham was buried with his wife Sarah. After Abraham's death, God blessed his son Isaac, who then lived near Beer Lahai Roi." Genesis 25:1-11

Narrated Abu Huraira:

Allah’s Apostle said, "The Prophet Abraham migrated with Sarah. The people (of the town where they migrated) gave her Ajar (i.e. Hajar). Sarah returned and said to Abraham, "Do you know that Allah has humiliated that pagan and he has given A SLAVE-GIRL FOR MY SERVICE?" (Sahih Al-Bukhari, Volume 3, Book 47, Number 803)

Narrated Abu Huraira:

Abraham did not tell a lie except on three occasion. Twice for the Sake of Allah when he said, "I am sick," and he said, "(I have not done this but) the big idol has done it." The (third was) that while Abraham and Sarah (his wife) were going (on a journey) they passed by (the territory of) a tyrant. Someone said to the tyrant, "This man (i.e. Abraham) is accompanied by a very charming lady." So, he sent for Abraham and asked him about Sarah saying, "Who is this lady?" Abraham said, "She is my sister." Abraham went to Sarah and said, "O Sarah! There are no believers on the surface of the earth except you and I. This man asked me about you and I have told him that you are my sister, so don't contradict my statement." The tyrant then called Sarah and when she went to him, he tried to take hold of her with his hand, but (his hand got stiff and) he was confounded. He asked Sarah. "Pray to Allah for me, and I shall not harm you." So Sarah asked Allah to cure him and he got cured. He tried to take hold of her for the second time, but (his hand got as stiff as or stiffer than before and) was more confounded. He again requested Sarah, "Pray to Allah for me, and I will not harm you." Sarah asked Allah again and he became alright. He then called one of his guards (who had brought her) and said, "You have not brought me a human being but have brought me a devil." The tyrant then gave Hajar AS A GIRL-SERVANT TO SARAH. Sarah came back (to Abraham) while he was praying. Abraham, gesturing with his hand, asked, "What has happened?" She replied, "Allah has spoiled the evil plot of the infidel (or immoral person) AND GAVE ME HAJAR FOR SERVICE." (Abu Huraira then addressed his listeners saying, "That (Hajar) was your mother, O Bani Ma-is-Sama (i.e. the Arabs, the descendants of Ishmael, Hajar’s son)." (Sahih Al-Bukhari, Volume 4, Book 55, Number 578; see also Volume 3, Book 34, Number 420)

And verily, there came Our Messengers to Abraham with glad tidings. They said: Peace! He answered, Peace! and he hastened to entertain them with a roasted calf. But when he saw their hands went not towards it (the meal), he felt some mistrust of them, and conceived a fear of them. They said: ‘Fear not, we have been sent against the people of Lot.’ And HIS WIFE was standing (there), and she laughed. But We gave her glad tidings of Isaac, and after him, of Jacob. They said: ‘Do you wonder at the Decree of All鈎? The Mercy of All鈎 and His Blessings be on you, O the family [of Abraham]. Surely, He is All-Praiseworthy, All-Glorious.’" S. 11:69-73

"Has the story reached thee, of the honored guests of Abraham? Behold, they entered his presence, and said: ‘Peace!’ He said, ‘Peace!’ (and thought, ‘These seem) unknown people.’ Then he turned quickly to his household, brought out a fatted calf, And placed it before them. He said, ‘Will ye not eat?’ (When they did not eat), He conceived a fear of them. They said, ‘Fear not,’ and they gave him glad tidings of a son endowed with knowledge. But HIS WIFE came forward clamoring: she smote her forehead and said: ‘A barren old woman!’ They said, ‘Even so has thy Lord spoken: and He is full of Wisdom and Knowledge.’" S. 51:24-30