Work for college affecting assignment
While the comments already provide some advice, let me add a bit with some possible options. First, you should be properly compensated for your efforts. Sometimes that can include course credit. Sometimes it can just include relationship building with a professor - looking toward the future. Compensation need not be monetary in all cases. This is likely why you did the project in the first place. So, you might examine what it is "worth" to you to do a favor for the university, or just for this professor. 

But there are other options. Telling the professor what your normal rates are for such work is valid. If you need that income to live and the time the project would take would lower your income if not compensated, then he needs to know that. 

But there might be a possible way to just step away from the project. For example, you could suggest that some other student(s) might benefit from carrying on the project and may not need to be compensated financially for their work. The experience itself can be valuable for them, where it is less so for yourself. 

But, if the university really needs professional level work done on this project, then they should expect to pay professional rates to get it done. The fact that you are also a student, in addition to being a professional, shouldn't give them reason to exploit you. 



I'll add, however, to address your main concern, no ethical person would blackmail you into doing something by holding grades over your head. If that is how the university behaves you should work to step away as soon as possible.