According to the Herald the Tory chief has reportedly failed to fully record his MSP salary, as well as almost £7,000 in outside earnings as a football referee.
He also still pulls in a salary from his role as a Conservative MP at Westminster.
In a statement to the paper, the Moray MP said: "This was an error on my behalf that shouldn't have happened, and I apologise for not registering these payments on time.
"Since realising my mistake last week, I contacted the Office of the Register of Interests and made them aware of the situation. All payments have now been declared, including those from my MSP salary that are donated to charities."
It comes asvoters in the UK are shifting support away from the Tories, due to ongoing sleaze scandals.
A poll by Savanta ComRes put Labour six points ahead of the Conservatives.