Under what conditions could slavery be morally acceptable?

There was a form of slavery in the Old Testament that God did not regard as morally wrong, but you had to treat the servant the way indentured servants were treated, with rights: the right to earn money outside their service, legal representation, care for their children's health and schooling, and so on. If all those things are satisfied, so the servant has everything the ordinary citizen has and is protected by the owner, then it is acceptable. What slavery became, however, was the stripping away of all those rights, so the slave was forever indebted, bound by literal chains, unable to escape, with no freedom at all; that is what is wrong with it. There is nothing wrong with putting yourself under the employment of a benefactor who pays you, which is what we do in corporate America, obeying their rules under their thumb. But you keep the right to leave, and a slave did not. If someone owes work or money, then you give him a contract spelling out what he must do to be free, so there is always a way out.