You're a volunteer, while I no longer oversee volunteers in a past job many of our contributors were students volunteering their time to effectively be able to stamp their name on something in use in the real world for purposes of their resume.
With all volunteer work there is VERY high turn over. (People's life situation changes, some days money's not that big a deal, other days it's everything) When the time comes a volunteer needs to part ways earlier than expected honesty is the best way.
Coming up with some lame excuse, or just cutting and running is a bridge burned, but if you take the time to express that you've valued your time together, and that the change is because you're in more immediate need of dedicating time to an actually income in a respectful manner, it's unlikely they'll be offended or upset with you. (perhaps the situation, but if they've done this for any length of time they'll see countless other volunteers come and go)
On the rare case if you explain you need to back out to draw in income I've seen that result in people getting hired. Still, the big thing here is to make it clear you're not unhappy about the situation, rather there is a necessity to pursue an increased income.
It's always possible they might hold leaving early against you, but in my experience when dealing with volunteers that's more the norm than actually having people put in the full amount of time. (hard to justify working for free when someone offers to pay you)