Have a Tax Question? Ask a Tax Expert
Welcome to Just Answers, I will do my best to Answer your Question.
I have to admit, that your question has me stumped but I believe that the 1042-S form can be used to file for the income so you would pay taxes here in the US instead of Milan.
Please view the link and see if this would satisfy the request.
I have never seen a "Certificate" that guarantees a taxpayer in the US that he/she would pay future taxes.