This question comes up a lot but I'm still not sure what the accepted practice is - and no, there's no mention in the contract about this.

I produced a documentary for a city government, via a private firm (it was part of a larger contract that the private firm had with the city). We did good, it's finished, everybody loved it, we got paid, rah rah, blah blah. Now I got a request from another division of the city for B roll to use in developing another video (themselves, it doesn't involve us).

What is the general practice here? I'm sure they are asking for the footage for free. I get this request quite often actually after somebody sees one of our pieces. In this case though, they are "sort of" the client that already paid because it's all part of the same city.

Not enough information to answer you with any accuracy.

Were you a subcontractor to the production firm that had a contract with the city?

Did your contract specify your work was a 'work for hire'?

Did the company's contract with the city specify who owned the rights to the footage?

Even if they are "sort of" the same client they paid for the documentary that you produced they didn't pay for all the footage that went into it (unless that was in the contract which I am guessing it wasn't). If they now want to purchase some of that footage then it's up to you & them to negotiate a price. What is it worth to them? If they didn't get the footage how much would it cost them to shoot it themselves?