I indicated voluntary trading. You can negotiate the value of things. If the other party agrees then it is acceptable.

As to future pay. That too would be voluntary. It also might be wise to have an escape clause from a contract you agree to.

If someone voluntarily enters into it without coercion or force then it wouldn't be slavery. It might still be unwise, but each person should be responsible for their own choices.

Reality is some people would make bad choices.