Sforno on Genesis 21:30/ספורנו על בראשית כ״א:ל׳

1כי את שבע כבשות תקח מידי, this is similar to the procedure described in Ruth 4,7 known as חליצה, in which the widow of the deceased brother who has been refused levirate marriage by her brother-in-law removed a shoe from her brother-in-law. This transaction signaled mutual agreement. In the case of חליצה it signaled that the widow was now free to marry someone of her choice instead of being beholden to a surviving brother-in-law.

2In this instance, acceptance of the gift meant that Avraham had dug the well, owned it, as well as the land it was on.