"Not due" is a phrase that is often used in the context of payments or deadlines. However, there are times when you may want to express the same idea using different words. Some synonyms for "not due" include "early," "premature," "untimely," "unearned," and "inopportune." These words all convey the idea that something is not expected or required at this point in time but may be at a later date. Depending on the context, one synonym may be more appropriate than the other. For instance, "inopportune" may be more suitable if you want to express a negative connotation, while "early" may be more neutral.