What about if I bought the MacGuffin for £500 and you value it at £1000? This makes no difference to the formal structure of the scenario (note that me valuing the MacGuffin at £11 would change the formal structure: then I might do X even if I knew Y was impossible: I would still reject an offer of £10). Then my behaviour feels utterly reasonable, rather than vicious and blackmail-ly.

I suggest moving the parenthetical to the end of the paragraph. I had to re-parse that in programming mode before I interpreted the ambiguity in the last sentence correctly and I wouldn't have noticed my confusion at all were I not familiar with the distinctions you are trying to express.