I honestly don't see the issue with private donations. Either it's something you want the police to have or it isn't. So the system needs to be structured to prevent them unilaterally deploying things that the bulk of the populace might not approve of.

I agree that a blacklist approach doesn't work. But neither does an expenditure limit. A value added limit is I think just a roundabout way of expressing a whitelist approach? Which seems like the only sensible solution to me. Ideally all deployments should require case-by-case approval unless an ordinance is passed to blanket approve an entire technology class for a specific type of usage.

Yes a whitelist makes a lot of sense, I don't know why I didn't think of that. Seems like it would be doable to blanket allow office supplies, gas, uniforms, etc. and then declare that anything else, even if it's free, must be approved by the public. If it's something uncontroversial then you can just update the ordinance.

Donations would be less of a problem with this system in place, but I still think it would be irresponsible to accept a donation without a plan to pay for it in the future if donations stop, and I also wouldn't want officers to feel indebted to some rich guy.