Your state's workers compensation laws - each state is different and there is a 49 out of 50 chance that your state is not the same as mine. So can't help you there.
Federal FMLA -- if the employer and employee are eligible but generally FMLA is UNPAID leave, so the merit increase would occur when the employee returned to work. What you can not do is refuse a raise to someone BECAUSE they are or have been out on FMLA leave -- that would be considered retaliation
Possibly any state FML laws for your specific state
The employer policy on raises for anyone on any type of leave.