I have a T1198 statement and thus have to file my tax report by mail but if I owe an amount do I have to pay the CRA before 30-04 or I can wait for the reassessment ?

You should pay your taxes owing before (or on) April 30. If you don’t you may face penalties.

If your return is assessed and the CRA decides that you don’t owe as much, they will give you a refund.