Can a bill collector sell a debt after they've agreed to accept monthly payments on a settlement amount?

Has some digit ever began monthly payments upon a allotment agreement offering by a check collector, as well as after a gourmet still took we to justice for a full volume of a debt anyway? Or did a gourmet sell a debt to an additional pick up bureau preceding to your final remuneration date? Can they do this legally even if they’ve put it in writing?