Additional information needed to know would have to be about the special events that are causing people to give money. Were these payments random or scheduled or expected or what. Payments were gathered in a routinized highly scheduled event. Special giving events were higher than his salary. They knew that the church couldn't keep this guy if they didn't pay so it wasn't really voluntary. Likely more like a tip than a gift. This will be gross income considering that it is for his service.