Thanks again for the advice, @Mike-And-Jane0. If nothing else, it helps me validate my understanding of what happened.
And thanks for joining the thread, @Alex893 .
The surprising, not-agreed-upon payments were the main problem but there were other related ones.
I'm not sure what standard of 'duress' this case may meet, but I did feel like I didn't have a choice.
a) Over the course of my stay the host had repeatedly shown a bad temper and rudeness when discussing problems both with other people in the house as well as with me on one very unpleasant occasion.
b) I did ask him to acknowledged the charges through ABB and he refused.
So this was not a misunderstanding. He was intentionally ripping me off and trying to cover it.
I wasn't going to risk a confrontation with someone dishonest and unstable.
I did pay by depositing money directly into the host's bank account (that receipt is one of the pieces of evidence submitted to ABB).
Otherwise, if not physical harm, I was concerned that I would have been prevented to take my belongings out of the house, which included heavy, expensive professional equipment.
In case you're wondering "why didn't you report the bad temper and rudeness to ABB when it happened", I did. I received a tone deaf, template response to the effect of "try to talk it out with your host".
In case you're wondering "why didn't you leave the place earlier if it were so bad", the short answer is: due to unusual constrains in my life at the time that made such move at the very least a hard choice.
I have indeed left a review.