Don't let them manipulate you. Assert your policy that they are paying for the software as it is at the time of donation. I would donate with the same thought without any expectations of further development because of my donation. Only that the software has already helped me in its current state.
Exactly this. The "purity" of OSS development needs to be reflected in the "purity" of donations for this to work. Small donations should represent nothing more than a token of appreciation. Large donations, well... money shouldn't buy project influence, though that is easier said. It's a complicated matter of corporate sponsored engineers contributing both good and bad (influence wise) patches back to OSS, but people need to make a living somehow.
Just wanted to add that you as a developer put the work up-front so there absolutely should be no expectation that the donation is for future work but instead for the work that has already been done.
