Is Zakat on salary obligatory, and is a Hawl (one year) a condition for it, and is it permissible to postpone it?

Monthly salary is not subject to zakat unless it reaches the nisab on its own, or by combining with other money or trade goods, and a full hawl (lunar year) has passed while it remained at the nisab. If these conditions are met, then zakat must be paid promptly and cannot be delayed. Otherwise, zakat is not obligatory.