What government rule allowed states to buy lands they claimed to the west

Jackie went to the grocery store to pick up yogurt, bread, and apples. first, she picked up a hand basket for carrying her groceries, and then she searched the store. after finding what she needed, she stood in a check-out line. then, the cashier put her items in a plastic bag, and soon after, jackie left the store. as readers of this event, we understand that jackie paid for the groceries, even though it wasn't mentioned, because we are relying on a grocery store