You shall not turn over a slave who has sought refuge to his master… (Deuteronomy 23:16)
If a slave from another land runs away to Israel, he is not to be returned to his master, even if that master is Jewish. Instead, we have the runaway slave write a promissory note obligating himself in the value needed to purchase his freedom. He must pay the master this amount as he is able. In exchange for this, the runaway slave receives a document certifying him freed.
The reason for this mitzvah is that God desires Israel to be a place of freedom. Someone who runs away to Israel does so for a reason. (This is true even in modern times. The Israeli airlifts from Ethiopia represent the only time in history when people took black people out of Africa to free them rather than enslave them.) When a person runs to Israel, it is appropriate to free him so that he may serve God alone.
This mitzvah applies at all times, even today. It is discussed in the Talmud in tractate Gittin on page 45a. It is codified in the Mishneh Torah in the eighth chapter of Hilchos Avadim. This mitzvah is #254 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.