The problem sounds like contract agreements that do not
clearly define ownership. It can be written both ways,
1. where the clients owns what you created while employed
by them or 2. the client merely licenses the software
you wrote for him to use. Whichever side of the fence
you belong on, your contract should clearly define the
terms.

If you are using pre-written modules that you coded well
before employment of said client, I would make sure that
any contract you sign clearly states that you retain ownership
of your modules used.