There are two types of incentive leaves. One are those as provided under the labor code, and the other, are those provided by the employer/company.
Those provided under the labor code, are commutable, means if not used, they are convertable to cash. If the company/employer will not convert it, then they are liable.
On the other hand , those provided by the employer/company, are discretionary, it may be converted to cash if unused or it may not(depending on their policy), however, this incentive leaves should be known clear to the employee beforehand, or prior to his undertaking, so that at the end the employee will not expect.
To avoid confusion, you must determine what type of incentive leave did you question, is it the one as provided under the labor code or one as provided by your employer.
It must be noted and it is a common practice that some incentive leaves as provided by the employer carries with it the term "service" or they generally term as "service" as provided in their policy and yet it is not convertable to cash based on their policy.