Another head-scratcher from mrghd.
Today, one meaning of up front is to start with, at the beginning, before anything else. I think this is the sense in the first occurrence of up front. The speaker is saying that he was clerking in an office in the jail just before his escape.
In the second use of up front, I think he is saying that as soon as he completes his current sentence he will immediately have to face a detainer warrant from Maine. Therefore he will probably not be released but will remain in detention pending extradition to Maine to face a charge of forgery.