A more thorough explanation:

Solutio indebiti is a legal term that means paying someone money that you do not owe them.

• John accidentally paid his landlord rent for the wrong month. The landlord received the payment but had a duty to give back the money because it was solutio indebiti.
• Sarah received a refund from a store for a product she never purchased. She knew it was solutio indebiti and returned the money to the store.

These examples illustrate solutio indebiti because in both cases, someone received money that they were not entitled to. In Roman law, if someone received payment for a nonexistent debt, they had a duty to return the money to the person who paid it. This is to prevent people from taking advantage of others' mistakes or accidents.