I think their friendship is coming along nicely, they might soon even become "roommates".
I absolutely adore this manga, it's super hot, like, more than 99% of hentai doujins I've ever read, perfect balance of lewdness, emotional intimity and story for me, that's very rare, fuck the naysayers.
If Marika didn't like it, she could simply void the contract and not go to Aya's house, knowing what's up. Aya has no power over Marika, not even financialy since Aya isn't poor or anything. And about the constant "Dameeeeee...." you can hear through the text, you know Japanese are big into fetish like that, and I'm into it too and I'm tired of pretending I'm not. Hearing her tell no when her body and heart says yes is a pretty common thing in japanese and other fiction is it not? It's plain to see that's what the author is going for here.