Start by separating four figures: agreed cash salary, tax value of the benefit in kind, actual payment and any additional charge. Add the contract version and the documented vehicle use for each month.
Then read the remuneration clause together with its schedules, addenda and previous practice. Section 914 ABGB provides the interpretation framework. Section 15 of the Austrian Income Tax Act explains the tax basis for private vehicle use, but it does not replace a clear agreement on the cash entitlement.
You can follow current legal updates through the Brandauer newsletter. If the reduction is disputed, submit the contract, payslips and communications in chronological order. Mag. Bernhard Brandauer, Rechtsanwalt, can help identify the next sensible steps.