This unusual process is referred to as being a redeemer. This term means that someone is buying back someone or something for themselves. In this case Boaz was buying Ruth out of slavery.

4 1 Meanwhile Boaz went up to the town gate and sat down there just as the guardian-redeemer [1] he had mentioned came along. Boaz said, "Come over here, my friend, and sit down." So he went over and ...