You did not say which sauna in Sao Paulo this event occurred.
If this happened at Lagoa and if you are there again and that same garoto is present, you should tell the owner, Rodolfo, of the scam pulled on you.
If you let Rodolfo know what happened, the garoto will be expelled from the sauna and will not be allowed to return until Rodolfo decides when he can return.
Even if you believe the garoto's argument was valid in that you did not agree on a price in advance, it was not reasonable for the garoto to insist on being paid five times the normal rate. That is why Rodolfo should know what the garoto did to you, a client at his sauna.
If it happened at Fragata, you don't have much recourse as that place is not run the same way as is Lagoa.