Is my university degree considered null and void, and are the allowances and salary I receive because of it unlawful, given that I combined work with morning studies, thereby violating one of the admission requirements, and what are the consequences for me as a result?

There is no harm in working with the certificate and benefiting from the salary because employers do not stipulate that one should not be a student. As for absence through collusion with the direct supervisor without taking leave, if the supervisor is authorized to do so, then there is no harm. Otherwise, you must return the equivalent of those days to the responsible authorities or donate it for public welfare or to the poor and needy. We do not see anything in what you have mentioned that prohibits benefiting from the certificate or the salary if you perform the work as required. Violating the university's condition of not combining work and study is forbidden, and seeking forgiveness and regretting it is sufficient, and it does not affect the validity of the certificate or the salary taken in return for work, but sin is incurred by violating the condition.