Actually, I was rather surprised with how forthright they were about it. They could have easily said "he left to peruse other opportunities" and not leave me thinking about any budget problems/disagreements, beyond the standard "non-profits don't have a lot of money".

Where I was unclear, and where I didn't get a lot of follow up information, was on what the disagreements were about. Did he think they needed an expensive XYZ hardware solution when he could have done with a cheaper and/or open-source one? Or was the organization requiring him to do the impossible without giving him any resources? It sounded like whatever it was, it was pervasive through their relationship.

My interviewers were non-technical, and so after some prying I decided to save it for our next meeting.