The Padma Purāṇa lays out the Dharma of prostitutes. Prostitutes must follow a vow which purifies them of sins. This vow includes worship of Viṣṇu, as well as service of a Brāhmaṇa for 13 months:
Having thus worshipped Govinda, the lord of the form of Anaṅga, the lady having made an offering with sandal, flowers, and incense, and then having invited a Brāhmaṇa...
She should bear in mind that the best brāhmaṇa, who has eaten well according to his sweet will, is Kāmadeva meant for Rati. The lady should do whatever the best brāhmaṇa desires.
She, speaking with a smile, should offer herself with all sincerity (to the brāhmaṇa)....
Thus till the thirteenth month arrives, she should gratify a brāhmaṇa and then send him to his house
This (vow) I have especially narrated to you, since this is the duty that should be always performed by the prostitute
My question is, does this vow really involve having sex with a Brāhmaṇa? I thought Brāhmaṇas are not allowed to have sex with prostitutes.
Or does this mean Brāhmaṇas are allowed to have sex with prostitutes who are following this vow?
If that is what is actually enjoined, then it follows from the idea that the dharma for all castes involves serving Brahmins and the Lord through what is permitted for their castes. For example, Kṣatriyas protect Brāhmaṇas and Yajñas, Vaiśyas farm and acquire food for Yajñas, Sūdras serve brahmins, wives serve their husband as the lord, etc. So along those lines, this vow might have prostitutes serve Brāhmaṇas through sexual service. But I am looking for stronger descriptions of this vow to confirm this.