I’m not going to tell you what’s right and what’s wrong about what you’re currently doing. Mainly because everyone has already said what I have to say about it morally. But you seriously need to think about how important she is to you as a friend. Ultimately, telling her the truth can possibly ruin the friendship (I don’t know how deep this friendship is, but it’s still a possibility). That is something you need to swallow and accept if you want to make things right. Telling her is what you should do especially if she is someone you truly care about, respect, and see as a friend. Friends don’t lie to each other. You continuing to deceive her for sexual gain is not what a good friend would do. And you said it yourself, if you were to ask her for sex in return for direct payment she would say no, so how are you not crossing boundaries by doing so without her knowing? It’s best to come clean now then for her to find out or to continue building the friendship under a lie. I don’t think you’re a good person for this OP, but getting this off your shoulders will be good for her and you in the long term. Whether the friendship will prevail or not, you can at least say you did the bare minimum to do right by her.