Is it that different from if I sell you a Euro, a Yen, a Dollar, and a Pound? They are fungible items whose value fluctuates.
How about if I sell you scrap gold jewelry on eBay? It's value will change before you get it and neither of us really knows how much gold is in it when the transaction takes place.
How about if I sell you scrap gold jewelry on eBay? It's value will change before you get it and neither of us really knows how much gold is in it when the transaction takes place.