Retroactive application of collective agreement qualification allowance and calculation of interest on wage difference

The Federal Labor Court (BAG) reversed the Landesarbeitsgericht decision and dismissed the defendant's appeal, thereby affirming the plaintiff's claim for payment of the difference in remuneration between 2010 and 2012, with the final order specifying that interest accrues from December 4, 2012. The dispute centered on a collective agreement dispute where the plaintiff argued that the employer failed to apply a newly agreed sectoral bonus ("Zulage für besondere Qualifikation") retroactively or properly from the date of the agreement's conclusion, challenging the employer's calculation method and application date. The decisive legal basis involved the interpretation of the relevant collective bargaining agreement, specifically the provisions regarding the effective date and scope of the new qualification allowance, as well as the principles of statutory interest under § 288 BGB for monetary claims arising from collective agreement entitlements.