Good thing nobody is calling free will slavery. Human will is, however, inalienable. If I agree to work 5 days for 100 dollars, I can leave in 4 days with 80, if I am free.
Then there is not difference than pricing your labor in credits you yourself own and produce. Which is not controlled by those who control the money your labor is priced in,
If you break your promises of labor in a credit system, no one will accept your credit anymore and become bankrupt until you can return to good credit. Its all on you keeping truthful.