That's the problem, they won't reach out to them themselves. This happens a lot with EMI's too, only difference would be that most ask for supporting evidence from the receiver and not the sender. At the end of the day, fixedfloat is most likely pulling a scam here, afaik whoever is behind it is a private figure, so we don't really know who's running the whole thing. Selective scams also happen a lot with platforms like this one, freewallet does it too.
OP provided enough and adequate evidence from the receiver side that he's not guilty, that's probably why they asked for other thing, to stretch this case.
Update: I Needed to force customer that i will not make any updates or maintain website as per deal if he doesn't contact them and he did few hours ago i am CC'ed inside email to make sure non party pull scam on me and you all will be updated on situation
You sure such stunt that you pull won't affect your reputation much in the future? Would FF release the non-tainted fund? If they're willing to do this, I think it's a better [though you're still losing here] solution rather than forcing your first customer with that kind of "threat".