How can one pay Zakat on wealth for the past ten years for which Zakat was not paid, knowing that the salary amount was variable and not precisely known?

Zakat (definition) is obligatory on savings if they reach the nisab (definition) (85 grams of gold or 595 grams of silver) and a complete Hijri year has passed over them. Its amount is 2.5%. Zakat does not lapse with the passage of years. If you are unable to accurately calculate it for past years, you must estimate and approximate by utilizing your bank account records, and then disburse what brings certainty or strong presumption of discharging your liability.