Undoubtedly, Rollbit is not a scammer. First of all, you have done a mistake by sending the payment to another network. All the community knows that sending on other networks means lost funds.
Does the Casino support asking for money to recover lost funds?
Exchange/gambling sites might have access to those networks too. That happened to Rollbit too. In this case, if they can recover those funds then they may charge for that service. If it is free then a lot of time wasters/scammers/competitors may abuse it and keep busy handling this type of transaction. So the site must take a fee for this type of service.
@OP as the fee is more than your deposit just think that you have deposited correctly and you have lost on the bet as there was no guaranty that you would be able to win on the gambling.
