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.