What can I do?

What is the name of your state? MI

I work for a bank and we have a business relationship with the owners of the business we are leasing from. The owner wanted a check verified before he cashed it. The check was verified as being a good check at that time. The check was initialed by the employee. The check later came back with a stop payment flag. The owner is now holding the bank responsible because he cashed a check that was verified by the bank employee. This employee can be terminated for this. Is this possible? Can the employee be terminated because of this?

Yes, the employee can be terminated for this. In an at-will state (every state except Montana is an at-will state) the employee can quit for any reason at all and the employee can be fired for any reason except one prohibited by law (race, gender, age etc.). It may or may not not be fair but it is legal.