Are you somewhat came from US? Bitfinex is somewhat playing this crypto hostage game for a while now, there are lots of users who experienced such this and can't read any of those got resolve.
Better to get in touch with them regarding this and try to make a way on how you can get/make this proof of funding docs.
Damn, why isn't the authorities aren't doing a shit about this case if it was happening for awhile now, I would be livid if more people have got their money hostage and the authorities that should be the deterrent to this never bats an eye. I would suggest that OP should also file some case against this scum of company.