I believe Bernie qualified for secret service by then and they were supposed to handle liaising with local PD. So Sanders campaign is saying "talk to secret service" and secret service is saying "we didn't ask for loan PD support so we're not paying".
It’s a police protection bill. Sanders was already under secret service protection by that point, and the secret service was who coordinated all of his security. So Sanders’ campaign manager basically told the city to bill the secret service instead of Sanders. Because again, the secret service was who requested the additional officers.