Ausländersteuer, or “foreigner tax” - also called tax deduction at source - refers to a form of limited tax liability pursuant to the German Income Tax Act (§ 50a (2) EStG). It is not a special tax for foreigners but rather refers to the obligation of persons living abroad to pay income tax on income earned in Germany. Since the nonresident taxpayer cannot be taxed in Germany, the domestic contractual partner is responsible for paying the tax (see also Payment debtor). The tax paid in Germany is considered deductible, depending on existing double taxation agreements.
Source: touring artists Einkommensteuer, edited