I'm also a researcher, but I've had to learn how these things work :) When you accept funds for a grant, that's a contract between the funding agency and typically your employer (UC Berkeley in my case), not you as a person. It's a contract to deliver the outcomes that you specified in your proposal.

A donation is just that, a donation: no strings attached, it is made in support of recipient's work and mission, but without any specific task in mind. Just like when you donate to your local public radio, you can't say "I want these funds to pay only for such and such program I like".