According to article 77 of the Code of Obligations, a person who has enriched himself without just cause at the expense of another is obliged to make restitution. 

For example, if a person transfers an amount of money to an account of third person by mistake, instead of the counterpart of the contract he made with; third person is under a liability to pay it back. There are four conditions for unjust enrichment to exist:

1. There must be enrichment in the property of a person.

2. There must be a decrease in the value of property of another person.

3. The enrichment must be without a just cause

4. There must be a causal relation between the enrichment and the decrease in the value of property.