What is a receipt?
A receipt is proof that payment was received after a sale. It lists what was paid for, confirms the payment amount, and shows the date the money changed hands so everyone can balance their books.
For example, when Clara pays John Hotdogs the $15 invoice on March 10, John gives her a receipt dated March 10 that itemizes the three hotdogs and marks the balance as paid. Keeping that receipt lets Clara track expenses and shows John the invoice is settled.