This is an interesting situation. First, I agree with cforde. If there are any contracts that state that they may have a right to this software, then unfortunately your hands are tied. But, if this is not the case then there are possibly a few options open. Some of these options are:

• They pay for the software and license
• They purchase a license agreement to utilize this module in their new environment
• You open open the code on CPAN or what not and they can d/l and configure it to meet their needs

More than likely, if they are requesting the code and all the templates - they will most likely get around and gather their own relationship with the Third Party software. I'm assuming (I know, bad habit) that their Tech. Dept. must understand what needs they have in order for their system to run properly. You could always just hand them the software, that is another choice. If you do that, will you be able to use it for another client(to solve possibly a future problem)? Or will it be legally theirs to do as they please with it? This may or may not be an issue. You did mention that giving the software to the client would promote open-source - but another question would be.. would it remain open source? The way it sounds to me, is that the module that you've written is proprietary, in which you utilize it to improve and meet your clients needs. Maybe the question is, do you wish to open it up to everyone? to just your clients? or sell it? Although, it would be great if there was an open source module for many banking companies out there. :)

-Moon