Wieviel darf ich als Deutscher innDeutschland arbeiten  

Es gibt spezielle Bescheinigungen bei Entsendung ins Ausland, siehe http://www.ccss.lu/pdfligne/Inst_E101_D.pdf

Aber vermutlich ist das bei so kurzen Zeiten im Ausland nicht notwendig. Am besten mit www.ccss.lu abklären!

So sieht meine Firma das:

Basically, because you are resident in Germany but working in Luxembourg, the Lux-Germany treaty comes into effect, which states that Germany claim the first right to tax you BUT, if your work days in Lux are over 183, then you automatically file and pay tax there instead.

However, the minute your work days in Luxembourg dip below 183 and your work days in Germany become more than incidental, then Germany operates their first right to tax you and you become taxable in Germany.

Bottom line -ensure you only have incidental days in Germany (a day here or there, not actual work days) and keep them to a minimum, and keep your work days in Lux over 183 days.

bis zu 183 Tage