Genesis 21:32

‘So they made a covenant at Beersheba, and Abimelech rose up, and Phicol the captain of his host, and they returned into the land of the Philistines.'

The treaty having been satisfactorily concluded the pair return to their land which is called ‘the land of the Philistines (see above prior to verse 1). In a sense, of course they are already in the land of the Philistines (Genesis 21:34) but the differentiation is made to demonstrate that now this part they have left is under Abraham's jurisdiction, with their agreement. We may possibly differentiate between the land actually occupied by the Philistines and that over which they have final control.