I'm gonna assume you're talking about a girl coming up to you randomly at a bar and asking you to buy her a drink. Definitely don't act butthurt about it and say you don't do that and I definitely wouldn't use the "I'll buy you a drink if you sleep with me" line either.
What I would do is give her shit about it in some way. Tell her to do a little jig for it. Ask her to tell you a joke or give her a trivia question to answer. Dare her to do something like go up to someone and say something stupid. Tell her "sure but I'm only buying you a drink if you can chug it in 2.4 seconds" or something like that. Say some shit like "I might, depending on your favorite music/movie/actor/whatever". Tell her you will if she asks the bartender a personal or embarrassing question. Like someone above said, tell her to drop and give you 10. Play rock paper scissors for it.
Just use it as an opportunity to tease and game her if she's attractive. In this situation she is essentially opening you. No need to get defensive or say something she'll think is creepy.
What I would do is give her shit about it in some way. Tell her to do a little jig for it. Ask her to tell you a joke or give her a trivia question to answer. Dare her to do something like go up to someone and say something stupid. Tell her "sure but I'm only buying you a drink if you can chug it in 2.4 seconds" or something like that. Say some shit like "I might, depending on your favorite music/movie/actor/whatever". Tell her you will if she asks the bartender a personal or embarrassing question. Like someone above said, tell her to drop and give you 10. Play rock paper scissors for it.
Just use it as an opportunity to tease and game her if she's attractive. In this situation she is essentially opening you. No need to get defensive or say something she'll think is creepy.