I lost the name of a story on mcstories about a college boy who helps a girl with a sprained ankle, and while helping her relax, her own want for him to hypnotise her ends up hypnotizing her in her dreams into being his sex slave, with him thinking it's just roleplay. If anyone knows it, let me know.

I don't know if it matches all the details you've given in the post (I know that it doesn't match-up with at least one because it's about a knee surgery instead of a sprained ankle) but I have something similar, posting for the slight possibility that you'd forgotten the details and actually remembering this one instead:

mcstories.com/Crutch/index.html

I think that’s it, I read his description and I can confirm besides the sprained ankle/knee surgery everything else matches up (I thought I remembered a bit about the girl getting hurt since it’s winter idk). The subject writes their rules at the start of everyday and delivers it to the protagonist so he’s in on the loop without the full context. Would recommend personally, good story.