If I were you I wouldn't worry about doing anything about the code being on Github. You need a lawyer to get the money you're owed, and then find another job.

As far as the bank goes: this is what they get when they outsource their work to an unscrupulous company. The company you're working for likely cheats both ways - don't expect to be treated any better in the future. They probably screw a lot of their employees over. They seem to have no issue doing illegal things. You don't want to be working for a company like this.