green paper

Today, we were buying some very necessary caffeine for Brett and myself, when we were asked for some green paper from the back seat. Brett looks at me; he and I have no green paper. Graham loves to hold on to the coupons and or receipts that are given when you buy something. Since the receipt is usually white paper, I had no idea what he was asking for. Then, light bulb clicked; he wanted the money we had used to pay with. Mind you, we usually never have cash, so Brett and I were like, how did he know about this? Brett looks at me and I say just give him a dollar to hang on to for now. He took it with him to the store, and then handed it to me to give the cashier when we were checking out!