Case C-20/16. Free movement – Tax

Does Article 39 EC (now Article 45 TFEU) preclude a provision of German law according to which contributions to the French pension and health insurance fund paid by an employee living in Germany but working for the French civil service — in contrast to comparable contributions paid by an employee working in Germany to the German social security fund — do not reduce the income tax basis of assessment, if, under the Convention between Germany and France, for the avoidance of double taxation, the salary may not be taxed in Germany and only increases the tax rate to be applied to other income?

Should question 1 be answered in the affirmative even if, within the framework of the taxation of the salary by France, the insurance contributions in question — whether specifically or at a flat rate — are taken into account as reducing tax, or could have been taken into account in reducing tax, but were not the subject of a claim to that effect and therefore were not actually taken into account?