Check in your abs quota config V_T559L - abs quota types (or V_556A_B), if you allow negative entl balance(i.e., Negative deduction field QTNEG).
Also for the absence type's deduction rule, check in V_T559L - Deduction rules (or V_556R_B), for the employee's groupings, there may be more than one abs quota type being used for deducting the absence hours,