Following the match, Mackay approached referee Anthony Taylor about the incident, but prior to viewing the replay.
"I've got to be careful what I say, because I'm certainly not intending having money taken from me," Mackay said.
"I did go in to ask for clarification of the ruling and what the referee and linesman saw.
"The explanation leaves me really disappointed, because I've since looked at it and the explanation was that they felt the goalkeeper dropped the ball.
"My goalkeeper bounced the ball. He saw Samuel Eto'o standing next to him and he bounced the ball.
"They (the match officials) thought he didn't have the ball in his control and he dropped it and the ball was in open play. That's a mistake."
Mackay asked if a foul would have been awarded if Marshall had been bouncing - and therefore in control of - the ball and was told yes by the officials.
Asked why Marshall chose to bounce the ball with Eto'o lurking, Mackay said: "Because he's allowed to bounce the ball and Eto'o's not allowed to touch the ball."
Mackay added: "(Marshall) said he saw Eto'o standing next to him and he knows the rule. He knows he's allowed to bounce the ball and that's why he bounced it."
Chelsea went on to claim an emphatic win, with Eto'o scoring his first Blues goal and substitute Oscar and Hazard adding to the score.
But Mackay felt the equaliser was a significant moment.
The Scot said: "It is a large factor. We were beaten (4-1), but I don't make any apologies for being beaten (4-1) rather than (2-1).
"At 2-1 we had to go to try to press and get back into the game. I felt at 1-1 we were excellent value for the way we were playing."
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