Hi,
You would need couple of rules to solve your requirement.
1.You would be getting a time wage type with number of leaves applied by EE .In Payroll schema check whether the number of days exceeded 15 or not if it exceeds then push this into your annual allowance leave wagetype if not do not process.
2,Once this rule is processed using Annual allowance wagetype function check whether any CRT value for annual allowance exists or not.If it exists then drop current month value otherwise pay it
Regards,
Uday