Commentary for Genesis 21:30

בעבור (supply the word זאת) — IN ORDER THAT this (where this, a feminine form in a neuter sense, as is usual in Hebrew, refers to the incident just recorded).

כי 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.

ויאמר...תהיה לי לעדה, “this gift that I am giving you shall serve as proof for me.”