They met after years and had lunch together. They remembered their old days and common friends. He paid the bill for the food but forgot to collect the receipt. She pointed it out and he casually reminded her that it won’t fetch him any reimbursement.
Later when they had coffee together, she paid the bill and promptly collected the receipt.
Did he ever wonder why?

Just so that, whenever she sees the receipt, the memory of spending some time with him would keep getting reimbursed.