Basically this is the 'fallout' from his 'skipping' the one debate to do a charity fundraiser in early 2016.
Under New York Law, as a New York resident, that was against the law. Trump isn't the first one to run afoul of that law, just the first one where millions were involved. In fact, the Clintons and others have done similar fundraisers (actually, far less beneficial**), but as non-New York residents. Although Hillary herself quickly had issues when she became a New York resident (when she carpetbagged), backing out of several 'last second.'
The 'bigger story' of this whole Trump episode is rather sad.
Trump gathered over $6M pledges, and let the donors designate their charities, not Trump's. The Trump Foundation just assisted in the 'paperwork' and 'relationships.' Unfortunately, with the US Media bashing, and 'shaming' of any companies and individuals donating, as well as the charities involved. So it was first ended up being cut to just over $4M, and then finally only $2.8M, although Trump cut over $1M ($1.2-1.3M I believe) of his own money (not the charity) to bring it back up over $4M total.
In fact, the issue with 'cutting checks' and other 'logistics' was due to the US Media's demonizations and shamings. It was completely sad, and really pissed me off.
Now the final insult is that only because Trump is a New York resident, Trump is going to be on-the-hook for the other $2.8M as himself. Kinda sad. Also explains why he finally moved to Florida. This is one episode where Trump actually did something really good, far better than any other politician I've evern seen in decades. The US Media not only skewed him, but cost veterans charities nearly $3.5M of the over $6M originally (although Trump made up for about $1.2-1.3M of those original pledges lost).
It was a genuine, heartfelt move that he did, and I wish more politicians did similarly. But New York law is New York law, and Trump, as a resident, screwed up in the end, despite the law that doesn't apply to most anyone else who is a resident of the other 49 states, DC, let alone other territories. I mean, in DC, this is 'every day' type BS, politicians and charity. So now he's on-the-hook for another $2.8M. Heck, Trump even used to do these for the Clintons!
**SIDE NOTE: Again, a lot of politicians do a lot of charity + fund raising (for their own campaign), and even Hillary -- who lambasted Trump -- quickly got called out by non-partisan charity groups that skewered her, and her husband, for doing a lot of golf and other, far less 'beneficial,' charity events for veterans (let alone some with Trump in the past). Make no mistake, the supermajority of the veterans charities -- named by the donors (there were originally around 2-3 dozen) -- were highly regarded, rated nearly all 3, 4 and 5 stars (out of 5) by independent, veteran groups, that the event raised money for.