Rule 631 Money delivered by one to another is presumed to have been due to the latter.

If you hand money to someone, the law assumes you owed it to them. In other words, courts will treat a payment as something the person had a right to receive.

This matters if there is ever a dispute about why money changed hands. The person who paid cannot simply claim the payment was a mistake or a gift without proving it. The burden falls on the person who made the payment to show the money was not actually owed.