Is it true that someone won 0.3 BTC from 0.01 and they hold the withdrawal in the name of investigation? LOL this is bad for a new Casino site. Except for the fact that you have not provided a proof that you really hold 50k BTC. The suggestion of one of the reply here is easy to do unless you really do not have that amount.
To sign a message which need multi-sign is difficult
You already asked for an easy way to prove your bankroll which I provided:
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A far easier way would be to send the min amount of dust to 14sMJzh8wB1MLbnuN2LvPUsfakomVY5h4c (a random address I just generated) and if we see the source of it was an address with 50,000 bitcoin we'll know you control it =)
Or an alternate way, would be generate a new multisig address, preannounce it here and then move all your funds there. That should be enough to confirm you have the funds