NYT reports part of the investigation into Cohen is that he facilitated a $150,000 payment to Trump's "foundation" for a video appearance and Q&A at a conference in Kiev in late 2015.
So... there's your big-deal campaign finance violation. Federal candidates are generally not allowed to take speaking fees or any sort of honorarium unless it's strictly unrelated to their campaign. In an advisory opinion issued relating to, of all people, David Duke when he was running in 1992, the FEC basically said they'll only allow it if there was no mention made of his then-ongoing presidential campaign. Whereas if you watch Trump's paid appearance via satellite in Kiev, it's all about his candidacy and campaign. It's the kind of typical appearance candidates make all the time, albeit not often for foreign audiences.
Candidates can't get paid by people to do interviews or make campaign speeches. Candidates who make money on the lecture circuit generally have to stop doing that when they formally declare their candidacy, because for most it's effectively impossible to separate their candidacy from it. You can't go taking six-figure checks from people in exchange for doing things that are inherently part of being a candidate. Particularly not from foreign nationals.
Under the way the FEC interprets the law in this area, this is essentially the equivalent of Mr. Foreign Billionaire giving $150,000 to the Trump campaign (illegal both as a foreign national, and also as vastly over the individual donation limit) and then Trump taking that money and personally pocketing it (also illegal, because of course it is).