I pissed him off enough that I actually got a reply.
I received it first thing this morning and it isn't pretty.

"this is bullshit amy! do not bother bringing the key back. i am not 100% sure would not have made a copy. i will be changing the locks monday. i do not know now what we would have to talk about. keep the money, i will be fine. as for yourself, did you do the right thing?"

I did not reply.

After work I had to run a few errands and then I went over to the house. He was sitting right by the open front window. I knocked, he hollered come in. The locks had not been changed. Yet, I suppose. His back was to me and he did not turn around. I asked if D11 was there and he said she was swimming at the neighbor's house. I told him I brought his money back and he told me he didn't want it (mean voice). I felt like crying but I didn't. I sat the money in the magazine basket beside his chair and said "But Jeff, it means something to me to give it back to you. Thank you for loaning it to me". Then I said goodbye and I closed the door behind me and left. I didn't give him back the key. I figured the locks were changed by then. I never would have made a copy of that key, though. Never.