We are thinking of building our own invoice system. I know you shouldn't re-invent the wheel, but we have very specific requirements. However, there may be open-source or sample implementations of invoice systems that we could start off with.
However, in the case that our invoice system works well for us, we might consider selling our system to customers in the future. Therefore, the licensing term of the system, if any, must be flexible enough to allow additions, changes as well as selling the system.
Does anyone have any knowledge if there are any such systems?