Every purchase with cash, including gas & food, requires a receipt – an actual receipt from the place of purchase, not a handwritten receipt by the salesperson who spent the money.
When the money is taken from petty cash, you must put a petty-cash-out slip in the box saying who you are, how much you took and what it’s for. When you return with your purchase, remove the petty-cash-out slip and replace it with your receipt (clearly marked with date, amount, and what was purchased) and the change.
Anyone who makes a purchase for cash and doesn’t turn in a receipt will pay for the purchase out of their own pocket.