Cession

The act of cession is the transfer of property to another entity (person, country, etc.). In international law, it commonly refers to land transferred by a treaty. It is different from annexation, where property is taken by force. Cession is voluntary or is supposed to be.

The Simple English Wiktionary has a definition for: retrocession.

Retrocession is to return something (e.g., land) that was ceded.

• Border dispute