John Calvin Commentary Genesis 21:28

"And Abraham set seven ewe lambs of the flock by themselves." — Genesis 21:28 (ASV)

And Abraham set seven ewe-lambs of the flock by themselves. Moses records another main point of the covenant: Abraham made specific arrangements for himself concerning the well, ensuring he would have free use of its water.

He then placed seven lambs in their presence, so that the king, upon receiving this honorary gift, would approve and ratify the digging of the well.

For the local inhabitants could have started a dispute, arguing that it was not lawful for a private individual, and a foreigner, to dig a well. But now, with the intervention of the king's public authority, Abraham’s peace was secured, so that no one would disturb him.

Many interpret 'lambs' here as pieces of money minted in the shape of lambs. However, since sheep and oxen have already been mentioned, and Moses now immediately adds that seven lambs were set apart, it is absurd, in this context, to suggest they were money.