A huge chunk of it was state spending, or the state agreeing not to force the city to meet state mandates:

Earlier this year, Ms. Hochul committed $1.5 billion in state aid for a host of municipal services. The state budget, which has not yet been finalized, is also expected to include a host of policy changes and revenue increases that will funnel another $4 billion to the city over the next two years.

...

The city is expected to save another $1 billion over two years from several changes, including the state’s expected agreement to delay a class-size mandate in public schools (despite Mr. Mamdani’s support for the mandate as a candidate); more school aid from the state; and the assumption by the state of a larger share of death benefits for families of police officers, firefighters and emergency medical workers.

That's happening because "Ms. Hochul, ... is facing re-election this year, [and] needs Mr. Mamdani’s help in turning out Democratic voters in New York City." After she's securely in office for another 4 year term, who knows how willing she'll be to do those same deals.

If Mandami remains popular, maybe he can keep the governor on his side. It would be ideal if they solved some of these problems as a team instead of working against each-other. I'm hopeful. Right now people seem to be in a very anti-status-quo mood. Mandami is showing how Democratic Socialism is a different way of doing things from the status quo. If he appears to be succeeding, more people will want to work with him. If more people can work with him, he can maybe take on some of the really big challenges that require a lot of people working together. At that point, it's not just a budget that appears to be balanced if you shift the pension burdens into the future. It may actually be a budget that will really work in the long term.