What is "mukataba"

In Islamic law, a mukataba (مكاتبة) is a contract of manumission between a master and a slave according to which the slave is required to pay a certain sum of money during a specific time period in exchange for freedom. In the legal literature, slaves who enter this contract are known as mukatab. Although the owner did not have to comply with the request, it was considered mustahabb (praiseworthy) to do so. Mukataba is one of the four procedures provided in Islam for manumission of slaves.