The purpose of both the documents is to record accounting transactions in order to account for receipt and request for payments. Both of these documents are the source documents for a financial transaction. This agreement is usually in the written form, or can also be entered into verbally, and the transaction terms are written and recorded in a receipt and invoice. When you sell a product or a service, being a seller or a buyer, you enter into an agreement to exchange goods and services for money, and as a result, execute a complete financial transaction.