INDENTURED SERVANT – a servant who sold himself to a master for a period of time (usually 4 to 7 years) in order to pay for passage to another country; the contract was transferrable, saleable, and was passed on to heirs if the master died
Merriam-Webster Online Dictionary
indentured servant (noun) a person who signs and is bound by indentures to work for another for a specified time especially in return for payment of travel expenses and maintenance