Slaves were not paid. Slaves couldn't even own property legally. What you're thinking of is what's called a peculium, which is not just a feature of slaves but also of everyone inside a familia--within a Roman household the only legal property owner was the pater familias. All property obtained by any of his children was legally his own, as was the property of all his slaves. The pater familias could dispense a sort of stewardship of property to anyone in his familia that he wanted, though--that's the peculium. Peculia involved administration of assets legally owned by the pater familias. They could be quite large, including land and even other slaves--the assets could also be traded, used, and increased without the approval or even knowledge of the pater familias, though he of course retained the right to remove a child or slave from administration of that property at will. Peculia were therefore, for all intents and purposes, private properties in all but legal name. It's wrong to think of slaves as being paid, however. A common peculium of a slave was the administration of a business or estate, especially since the senatorial class was itself legally barred from business. In such a role a slave might be very really collecting wages from a business that was in all but name his own property--but he didn't keep that money, not in the long run. Either he returned the money he made from the business to his master or he kept it (or a part of it) in his own peculium until he could return the peculium entirely in exchange for his freedom. The way that slaves could purchase their freedom involves some odd legal formalities. Since slaves could not own property they could not technically have any money of their own to purchase their freedom. Instead, the requirements for a statuliber (a slave manumitted after the completion of a fixed legal condition, agreed upon by the master--legally speaking a slave could get all the money in the world and if his master just didn't want to manumit him for it he was shit out of luck) generally involved increase of the peculium according to a certain amount. The idea is that whatever money the slave is able to make, through business or through receiving gifts or whatever, goes into his peculium. It is, for all intents and purposes, his private wealth, but legally it belongs to his master and it will have to be returned to him if the slave is given freedom. Therefore while he's using the money it's essentially his, but the moment he purchases his freedom it returns to his master, who's now wealthier the same way he would be if a friend had simply paid him a gift--likewise the master could simply remove funds from a slave's peculium whenever he pleased.

But no slave was receiving wages for his work. Any money a slave earned with his peculium was done with the conscious knowledge that it was the master's money and that it was being, one way or another, earned for the master's benefit