Upon what terms is Douglass allowed by Master Thomas to hire himself out? Who does this agreement benefit most?

Douglass is hired out to learn a trade at which he makes a great living with hight wages (as a caulker). Unfortunately, although Douglass has learned a trade, it benefits his master most, as he receives every cent Douglass garners.

Note: Douglass is no longer with Master Thomas. He is with Master Hugh.