Make the bet more interesting.

“Hell if you aren’t afraid of loosing how about we change up the wager, if you win I give you the 500 bucks, and if I win you have to suck my cock and I get to fuck you up the ass like my little bitch.”

“What? Do I look like some kind of fag to you?” He balls up his fist and you think you are going to get punched.

You quickly interrupt him, “Hey I was just saying, if you are sure a shemale can’t win I would rather win that than the money. You are sexy as hell.” You bat your eyelashes at him seductively and rub your hand across your ass.

This seems to pacify his ego and he calms down. To your surprise he then seems to be actually be thinking about it.