It is already done in some restricted cases; I have a civilian buddy who still owes the Navy a few more years of work to repay his education. He has described

It is already done in some restricted cases; I have a civilian buddy who still owes the Navy a few more years of work to repay his education. He has described it as indentured servitude. Technically he could quit and pay back the Navy, but they've been paying him so poorly that doing so would plunge him into debt.