It's not as easy to donate a service dog as one might think, not to a PD anyway. First off, the dog has to have a very good foundation. Next they have to have someone that knows how to test a dog. Police dog handlers, god bless 'em, know how to handle a dog, they don't know about foundation, training, or testing. So they'd have to find somebody to do it. Even after that, they'd have to find somebody to finish the dog.
Foundation is one thing, finishing it is another, then you have to train the handler with the dog as well. Once a PD gets to that point, the usually just buy a dog from the place that trains them, one stop shopping so to speak. and if they already have a K-9 unit in place, good luck cracking into that. they already have somebody that imports, trains, and certifies the handlers and dogs together. With all the screw balls claiming to sell PD and PPD dogs and train them, once a dept finds someone they trust it's hard to break into that circle. Not saying impossible, buy you have to prove yourself and your dogs for quite some time, and even then, the higher ups usually don't allow it anyway.
But it is a noble thought, just don't expect to keep a dog for a year and then have depts begging for your dog