From Wikipedia:

A sale is the act of selling a product or service in return for money or other compensation. It is an act of completion of a commercial activity.

The seller or salesperson – the provider of the goods or services – completes a sale in response to an acquisition or to an appropriation[citation needed] or to a request. There follows the passing of title (property or ownership) in the item, and the application and due settlement of a price, the obligation for which arises due to the seller’s requirement to pass ownership. Ideally, a seller agrees upon a price at which he willingly parts with ownership of or any claim upon the item. The purchaser, though a party to the sale, does not execute the sale, only the seller does that. To be precise the sale completes prior to the payment and gives rise to the obligation of payment. If the seller completes the first two above stages (consent and passing ownership) of the sale prior to settlement of the price, the sale remains valid and gives rise to an obligation to pay.