How to set different negative balance limits based on Absence reason

Summary: We have a requirement to allow employees upto 80 hours negative when employees use specific Absence Reasons. Can anyone suggest the Absence Entry Validation formula?

Formula used but not getting complied

DEFAULT FOR ABS_EXT_PER_ACR_NEGATIVE_BAL_LIMIT is 0

DEFAULT FOR PER_EXT_ABS_REASON is 'ABC'

Neg_Bal_Limit = ABS_EXT_PER_ACR_NEGATIVE_BAL_LIMIT

Abs_Reason = PER_EXT_ABS_REASON

IF Abs_Reason = 'Covid Related'

then Neg_Bal_Limit = 80

else Neg_Bal_Limit = 0

Return Valid,Neg_Bal_Limit