I cornered the relevant housemate in the kitchen last night. Asked him if he had a minute. He asked what it was about.
"I don't want to hear it."
"Then we'll keep it short. [recap of my views.]"
"[recap of his views, his promise to pay, etc. Increasingly defensive.]"
[Interestingly, he claims to have been using the AC for its fan properties since the heat wave broke. Um...what about the dust? I didn't ask.]
"I'm still not OK with paying for your AC, which is what you're asking me to do until the money comes in. I need you either to agree to use the fan function only until then or to take it out."
We went around with this one for a bit. Eventually, we settled into a stubborn and unhappy silence. And then he left the room. I went after him, asking if he needed more time to think. He said, "No, I've given up on thinking." (I have no idea what that means.)
So...now what? I drafted an email, which I will send (if it still seems like I said things right) in a little while, saying that I need to hear from him soon, or that I will come into his room tomorrow after work to take the thing out. Which gives him the required twenty-four hours notice for me coming into his room for non-emergency purposes (something I have never really bothered with before, but which feels like a good idea in this situation).