Abimelech was asking Isaac to make an oath in covenant terms with him. Abimelech could see the power of the LORD and wanted to ensure that Isaac will do no harm towards him. Abimelech reminds Isaac that they (Philistines) have not harmed Isaac in any way and has done good towards him. And, sent Isaac away in peace.
"Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so" (Matthew 19:8)....
Not that I am speaking of being in need, for I have learned in whatever situation I am to be content. I know how...
“At the end of every seven years you shall grant a release. And this is the manner of the release: every creditor shall release...