I've finished and I'm very impressed with the puzzle. I don't like it when answers have to be entered jumbled but at least with this one there was a purpose to it.
I have to say, though, that for me at least, the title and the four extra words didn't help one bit. If anything, they slowed me down, or perhaps I got the wrong four words. I kept on thinking about an IOU; and while the construction is clever, it is not impossible, and there are many more examples to be found on the internet.