Jamieson, Fausset and Brown on Genesis 43:12

12. take double money--the first sum to be returned, and another sum for a new supply. The restored money in the sacks' mouth was a perplexing circumstance. But it might have been done inadvertently by one of the servants--so Jacob persuaded himself--and happy it was for his own peace and the encouragement of the travellers that he took this view. Besides the duty of restoring it, honesty in their case was clearly the best, the safest policy.