The Colombian prostitute behind the massive Secret Service scandal is finally admitting she had sex with one of the agents -- claiming he seriously short-changed her for her services, paying only $50 ... when she explicitly asked for $800.
24-year-old Dania Londono Suarez appeared on "Today" this morning with a translator -- claiming the agent was very direct about asking for sex when they met at a Cartagena bar last month.
Dania says she told him up front ... a night with her would cost $800 -- but the next morning, after they had had sex, the agent grew "very angry" when she asked for her money.
According to Dania, he threw her $50 and kicked her out -- and that's when police got involved.
The agent in question has reportedly resigned.