The FEC ruled it should be considered a campaign contribution, which is why both Cohen and David Pecker of the National Enquirer were determined to have violated campaign finance violations and were fined accordingly.

And this letter was intended to cover Cohen's ass, but actually revealed Trump didn't supply the payment, which is why they looked into his banking history and discovered he lied to obtain the loan.

As far as Trump paying Cohen after the election, the FEC ruled in Trump's favor, as he had employed Cohen for this exact purpose (killing unflattering stories) long before he even considered running for President.