Hmm, well I haven't dealt with this personally. However, any time you give a woman something you'd prefer not to give her, it's generally best to act like whatever it is you give her is no big deal. "Sure, here; no big deal. Just get it back to me in a reasonable amount of time."
Also, if you have a history where you give this girl money, then she stops going to bed with you, I'd call that out the next time she needed money again. "Hey, I'm going to give you this money, it's no big deal. But if it leads to you closing your legs to me because you feel weird about taking my money or whatever it is, then maybe we shouldn't do this anymore. We can take you to the bank or somewhere to get a loan there instead. I don't want to do something nice for you and have it hurt our relationship."
You can also be firmer about it: "No, I'm not doing that. Every time I give you money you turn into a prude and close your legs to me. I lose money AND I stop getting laid. It's the worst possible deal ever. There's no motivation for me to do that. No. Go get a loan from the bank." She'll protest, whine, complain, promise you she won't stop putting out; finally after she spends 10 minutes trying to convince you you can give in and say okay fine, but seriously, if you do it to me again you will never get another cent from me. I want you to understand that.
One note: be careful how you talk about sex. It's very important to not talk about it as something she gives to you. Or a reward for you. Or any framing like that. Instead it is just her doing things that hurt the relationship, or you having to go celibate, or her being weird and closing her legs. It is not about "I want sex but you're not giving it to me"; subtle but very important difference in how you frame the sex.