This article outlines the steps to correctly handle a requirement to void an agent check in the sales module in a previous calendar year.
Instead of voiding the agent check, which will have an impact on your previous and current year’s financials, set the agent up as a vendor.
Then create a negative manual bill for the amount of the check, referencing the check number you wish to void. Then enter a new unpaid bill for that vendor and issue them a new check. Code both the manual bill and the unpaid bill to your commission account. The two records will net to zero, so will have no impact on your previous or current year financials.