And (Joseph) told his servants to put their stock-in-trade (with which they had bartered) into their saddle-bags, so they should know it only when they returned to their people, in order that they might come back.
“Sayiidina Yusuf (pbuh) said to his attendants, ‘Place their funds (the price which they paid for the grains) back into their carriages (without them knowing) so that they recognise it when they return to their families, and so that they may return.” He knew that once they realises his generosity, they would soon return.
Madani, Muhammad. Anwar ul Bayan. (vol. 3 p. 74). Collection of Quran Commentaries (Tafsirs) – Islamic Web Library