Parkin will stay at Cardiff now and look to force his way into the matchday squad. He scored in a 2 - 0 win for the Development side over Neath on Tuesday. The visitors had Lee Trundle in their team.
Mackay said: "Financially it wasn't acceptable. It was a different deal from than he went on initially.
"There was no reason for that to change as far as we were concerned."
Mackay has confirmed that he is looking to bring in loan players.
He said: "I'm talking to a couple of clubs at the moment - whether they come off or not is another matter.
"But it's not been a particular busy loan window for anybody and there are reasons for that - the lack of players available that are going to tick the boxes you need. "