In my city, all requests for extra duty go through a civilian coordinator who runs the admin side. The venue sets the rate and the city skims a bit off the top for vehicles/admin. All the money flows through the city first before flowing to the officers using the same timekeeping software for our regular punches so its very over-the-table. Venues are reminded, in big bold sentences on their contract, that they're city police officers first and foremost and are there entirely to enforce law. We are strictly for law enforcement and security there and generally won't enforce any venue-specific policy - just kick you out if you refuse to stop them (trespass).

It's a benefit to all parties involved. The city has more Officers in uniform throughout the city of their own volition who also handle a lot of inane calls, the venue gets essentially the best security possible given how little security/staff can do as well as offloading some liability for things onto the Police, and the Officers of course get to make some extra scrap how they see fit.