A man seeking to purchase sexual services from a woman got more than he bargained for Thursday afternoon, when a third party interrupted their transaction and robbed the aspiring customer at knifepoint.

It was 4:27 in the afternoon when a 58-year-old man "brought a female to his apartment" on the 1000 block of Post Street "for sexual favors," according to a report from the San Francisco Police Department.

As the man was negotiating with the sex worker, a man described as around 26 years in age arrived on the scene "and demanded money" from the service-seeking man while wielding what police describe as a "pocket knife."

The victim turned over the dough, and the knifeman and the woman left the apartment together, fleeing in an "unknown direction," police say.

No arrests have been made in the incident, according to police, and the suspect remains at large.