The Appeals Committee of Super Rugby's governing body SANZAR quashed the fine which came after Byrnes attacked Carter in newspaper comments published in March.
The comments followed Byrnes' successful appeal against a 10-match ban for dangerous play against Carter who had alleged he was eye-gouged during the round two match at AAMI Park.
Byrnes hasn't played for the Rebels since that game after suffering an elbow injury and then being unable to force his way back into the squad.
Earlier this month he was released by Melbourne to return to Russia, who he played with at last year's World Cup, to be part of their squad for the IRB Cup that features emerging countries.
SANZAR said on Wednesday it had revised the sanction with Byrnes still receiving a formal caution.
He still has to pay costs of $2500.
The Appeals Committee said the decision to wipe the fine was due to the "very unusual facts of this case".
"It was the first of its kind, and should therefore not be seen as a precedent," SANZAR said in a statement.
"Players wanting to complain about other players should do so through SANZAR channels rather than through the media.
"If they do speak to the media they place themselves at risk of facing a charge of breaching clause 8.3 (g) of the SANZAR Code of Conduct. If established, such a charge will usually result in a substantial fine."