It’s not slavery because it isn’t the lump sum purchase of a person’s labor in perpetuity, anymore than buying food is slavery to the producers. Privately owning and then leasing land/housing is an unnecessary institution for distributing housing, but it isn’t slavery. The word has a sense to it of an extremely low category of servitude.
Wage labor is however the blatant institutional piecemeal sale of one’s labor. It is much closer to outright slavery, the wage contract grants the employer the right to direct your labor and implicitly (usually not outright declared, but it is obviously the case) appropriate the results. The wage contract claims the employer is merely buying your labor services and the right to direct them, but clearly the right to direct labor and appropriation of the results are separate things. This is also clearly seen in tort law, whereby an employee can be sued for a negligent tort (an industrial accident for instance) because they are considered ultimately responsible for results of actions outside the scope of the proper fulfillment of their wage contract. So the employer effectively has the implicit right to the results of all actions deemed reasonably beneficial to their business needs for the wage contract, but actions outside of that return responsibility to the agent. I think this is all pretty common sense outside of any legal argument (let alone among Marxists), but I make it anyways just because average people have a tendency to deny it out of indoctrination into the liberal order. It is common to believe that somehow the owner of the business truly does “create” the results, through their “entrepreneurship” or something, but it is a legal fact that the employment contract is the literal sale of ownership over the results of one’s labor. Whether the institutional convention of private money capital being required to fund startup costs results in some responsibility conferring to the capitalist is irrelevant to the fact that the capitalist purchases the right to responsibility for your labor from you, which is prior and continues to exist implicit in the fact that your responsibility for your labor returns to you when you either commit a negligent tort, a crime, or leave work for the day.