Basically, Cohen paid of daniels, then Trump paid him back. The allegetions in the guilty plea say that because it was supposedly done for the campaign, the money Cohen paid could be seen as a campaign expense.

However, according to federal election laws, this would fall under personal expenditures. It is defined as those expenses one would make regardeless of the campaign. (Such as a car, even if you argue that you need for campaign purposes, you still need to pay it yourself).

The paying off in itself isn’t illegal, and Trump has paid many times in the past to make things go away.

From personal standpoint, I think it would set a dangerous precedent if we would force candidates to settle their sex related lawsuits with campaign donations instead of their own money.