"You shall not deliver a servant to his master"Deuteronomy 23:16.

It is forbidden to return a non-Jewish slave to his master (even if he is Jewish) if he has escaped and fled to Israel. Rather, the slave must be given his freedom, and he must compensate his former master for his loss (i.e., his own monetary value). This because the slave has come to find refuge in the pure land that G‑d chose for His esteemed nation.