Adam Clarke on Genesis 21:23

Now therefore swear unto me - The oath on such occasions probably meant no more than the mutual promise of both the parties, when they slew an animal, poured out the blood as a sacrifice to God, and then passed between the pieces. See this ceremony, Gen 15:18 (note), and on Genesis 15 (note).

According to the kindness that I have done - The simple claims of justice were alone set up among virtuous people in those ancient times, which constitute the basis of the famous lex talionis, or law of like for like, kind office for kind office, and breach for breach.