Taxation of foreign income for G4 visa holder

[marcus82 0]

Situation:

• I am a German citizen working and living in the U.S. on a G4 non-resident visa (work for United Nations). As such, I am exempt from U.S. income tax related to my work at the UN.
• In addition to my part-time work in the U.S. for the UN, I work as a consultant for other agencies and companies based outside the U.S (this work is performed outside the U.S. as well).
• I am married to a U.S. citizen.

Questions:

1. In which country does my income from non-U.S. employers have to be taxed? In my home country Germany or in the U.S.? I understand that my income from German employers is taxed in Germany, but that other foreign income may have to be declared in the U.S. given that this is where I am based most of the year.
2. Provided that I would have to declare my non-U.S. income in the U.S., how do I determine the tax rate on the income from foreign sources? Is it automatically 30% because of my non-resident status or is there a possibility to avoid the flat rate?
3. I understand my G4 visa status restricts me from other salaried work in the U.S. that is not related to the United Nations. Thus, am I allowed to have a contract for a U.S. based employer for work performed outside the U.S. (i.e. home-based assignment outside the U.S.)? If yes, when filing a W-8Ben, do tax treaty benefits apply given that there is a tax treaty between the U.S. and Germany? In other words, can I be exempt of the 30% automatic tax withholding and under what conditions?

Thanks in advance!