If an agent has receipts for spending on meals for that day which is equal to or more than the per diem, this particular matter would be resolved.
If an agent does not have such receipts, they’d be open to the accusation that their per diem was used on that day towards paying the prostitute. Since money is fungible, IMHO, it would be extremely difficult to avoid such a charge without receipts proving that the per diem was spent on valid items.
IMHO.
Yes, money is fungible. So the question is pointless. Someone can “profit” off of his per diem by spending $20 on food where he’s allowed $35. That money is his to keep.
Now that it’s his money, whether he puts it in the bank or spends it on hookers, it’s still his money.