How about custom-coding their sponsored features into a proprietary module just for them, then taking some time to "make it right" before incorporating those features into the open source version? Of course, the contract would need to be clear that you will eventually release their sponsored code into the free version, but what they're paying for is to get it when they want it, and possibly with a custom I/O that they specify.
So basically, you'll have one official open source module, and a whole bunch of proprietary variants for which you are paid to maintain.