Genesis 21:23

And now, swear to me here, by the Word of the Lord, that thou wilt not be false with me, nor with my son, nor with the son of my son: according to the kindness which I have done with thee, thou shalt do with me, and with the land in which thou dwellest.

NOW THEREFORE SWEAR UNTO ME HERE BY G-D ‘IM’ (IF) YOU WILL BE FALSE TO ME. The word im always expresses doubt — do not think of it in any other way to mean “that.” See Ramban there, where he explains it in a manner consistent with his teaching here. — and it appears in most places in connection with an oath: If you will be false to me; And therefore I have sworn unto the house of Eli ‘im’ (if) the iniquity of Eli’s house shall be expiated; Once I have sworn by My holiness ‘im’ (if) I will be false unto David; ‘im’ (if) they should enter into My rest;, 95:11. And he was wroth, and swore, saying, ‘im’ (if) one of these men will see.
The purport of this is that since oaths are given with imprecation, Abimelech is stating, “Swear to me, saying, G-d do so to me, and more also if you will be false to me.” Likewise it is said, Let there now be an oath between us. And in the matter of a Divine oath: I have sworn by My holiness if I will be false to David; if the iniquity of Eli’s house shall be expiated, meaning “If that will be so, then My word is not true,” and in similar cases, since it does not want to expressly state the condition. Scripture modifies and shortens these expressions. A similar case of a shortened condition is the verse, And Jabez called on the G-d of Israel, saying: Oh that Thou wouldest bless me indeed, and enlarge my border, etc., and that Thou wouldest work deliverance from evil that it may not pain me. And G-d granted him that which he requested. Here the entire condition is missing. A similar example is the verse, If they will see the land, referring back to the first verse, As I live, and all shall be filled with the glory of the Eternal,, Verse 21. and Scripture shortens the Divine oath.
The expression, false to me, is because Abimelech was a king, and Abraham dwelt in his land. [Thus, if Abraham were to do him evil, it would be an act of disloyalty towards him in his royal capacity], or it would be a betrayal of Abimelech’s love for him, as he [Abimelech] was his trustworthy friend, honoring him and doing his will. For you see that Abraham found no fault with him except the well of water, which Abimelech’s servants had violently taken away, and the king said to him, According to the kindness that I have done unto thee.

ולנכדי, to my grandson.

כחסד אשר עשיתי עמך תעשה עמדי, do me the favour to include your children in the oath you swear to me.

ועתה..אם תשקור לי, perhaps the subject here is the immediate future, i.e. ועתה. Avimelech is concerned that within the next or subsequent generation the friendly relations between Avraham and himself might sour. Therefore he suggests a treaty reinforced by an oath as he is sure that Avraham would not renege on an undertaking confirmed by an oath. He is trying to commit the next 3 generations to this undertaking he asks from Avraham.

The words לי ולניני ולנכדי comprise three generations. The word נין refers to the son, whereas the word נכד refers to the son’s son. The same words appear in the same sense in Isaiah 14,22 והכרתי לבבל שם ושאר נין ונכד, “I will wipe out from Babylon name and remnant, son and grandson, etc.”

כחסד אשר עשיתי עמך תעשה עמדי, I allowed you to settle in any part of my country that you chose to settle in, and I respected you.

ועם הארץ אשר גרת בה, I expect you also to act in a friendly manner with all the inhabitants of this country who will survive me, both you and your children and grandchildren after you, seeing that you have been treated well in this country by its people.