Is it possible to buy sex with a slave from a slave owner? No.
No act of buying something may be immoral. If it is truly buying then it is not immoral. If it seems wrong then it is not buying, it is something else.
In your case giving your money to the slave owner in order to get his permission to rape the slave is immoral.
Giving your money to the slave owner in order to get his exclusive permission to rape the slave and then not raping it is not immoral, however, but may even be commendable since it ensures the slave will not be raped by anyone that night.