A report by El Mundo claimed the deal Messi signed at Camp Nou in 2017, which expires at the end of this season, is worth more than €555million.
It was already believed that Messi was on an extraordinary salary at Barcelona, given he is recognised by many as the greatest current player of the world's most popular sport.
The leaking of the exact sums payable to Messi has caused a stir, coming at a time when embattled LaLiga giants Barcelona are reported to have built up debts of over €1billion.
Koeman rejected any suggestion that Messi's contract is in any way connected to Barcelona facing a financial hurdle, insisting he has "helped this club to make it very great".
The club's record scorer, who netted his 650th Barcelona goal in Sunday's 2-1 win over Athletic Bilbao, could leave on a free transfer in a matter of months.
And Koeman vowed that if anyone employed by Barcelona was proven to have leaked private details, they would be shown the door.
"If you are part of the club, you have to help the club, the coaches, the players. If someone from the club has leaked this, they cannot have a future at this club," Koeman said.
"They are things that come out, that cannot come out. There are people who are interested in hurting Messi and Barca."
Messi is sure to have been unimpressed by his personal details making it into the public domain, but the record six-time Ballon d'Or winner did not let it impede his performance on the pitch against Athletic.
Speaking after the game, Koeman expressed alarm that anyone would want to cause upset in the ranks, particularly concerning Messi, a player destined to be remembered as an all-time great.
"You have to respect him, I see bad intention and desire to hurt," Koeman said, quoted by Mundo Deportivo.
"We must try to find out how this could have come out in the press and we have to focus on our work, that the team are well, and we cannot allow ourselves to lose focus over this issue."