Lawyers can pretty much set up any kind of billing arrangements they want and lawyer/client relations are sacrosanct (or at least were into Mueller decided to run roughshod over them).
President Trump (as he has repeatedly explained) employed many, many lawyers in various capacities.
It seems that for Trump, Cohen was sort of an "at-large" lawyer and perhaps all around fixer and someone who was paid on a retainer and/or through a trust account or escrow account. So what?
I would remind all of you of the following: 1) This all occurred BEFORE Trump was president. 2) The "lady" in question is a porn star -- not exactly terribly credible. 3) This pales in comparison so Clinton's antics (IN the Oval Office) and his crime of perjury and 4) This is really not within the scope of Mueller's investigation -- this is a wild goose chase!