Commentary for Genesis 21:32

AND THEY RETURNED TO THE LAND OF THE PHILISTINES. The sense of the verse is that they returned to their city which was in the land of the Philistines for they lived in the land of the Philistines. However, they abode in Gerar,³⁶⁰Above, 20:1. which was the royal capital, while Abraham dwelled in Beer-sheba, which is in the land of the Philistines, in the valley of Gerar.³⁶¹Further, 26:17.

וישובו אל ארץ פלשתים, they returned from Beer Sheva, i.e. proof that Beer Sheva was not part of the land of the Philistines. They had gone there only to speak with Avraham who had gone there to see how his livestock was faring. This is where he tendered the gift to confirm the covenant they had made.

ויכרתו...ויקם אבימלך, Avraham remained at Beer Sheva with his flocks for a day or two longer before returning to the land of the Philistines to his house in Gerar.

וישובו אל ארץ פלשתים, “they returned to the land of the Philistines.” This means that they returned to the capital of the land of the Philistines, seeing that they had never left the land of the Philistines, the covenant having been concluded on land which was theirs.