Goods and or money in exchange for sex is prostitution. The rest is in the details, or the law for good or ill. Not all slaves are prostitutes. Some mine copper, or harvest chocolate, or make shoes.
If this 'rest' is the difference between prostitution and slavery, it most definitely is not 'in the details'.
It's confusing the job with the status of the worker, whether they do the job by choice or force, the job is the same.
It's funny how bigotry will out in these discussions. I'm reminded of the poster who insisted that an abused 10 year-old who was was being given sweets for sex was irredeemably corrupt.