Salary increment

All,
One of our employee salary was revised in April 2019, but this was not corrected in the monthly salary payout nor did the employee notice until today. So now in May 2021, this needs to be processed and paid as “Salary in Arrears”. Am I correct, please clarify.
Secondly, the affected employee is asking us not to pay this as an “arrears” but include it in this year’s salary. Is this possible and how. Can someone clarify.

Thank you and Regards