Hi all,
Here is my question :
Carlo, an Italian citizen and a non U.S. resident, sends money to a financial company in the States. The company lends his money on his behalf to one of their customers. Does Carlo have to fill any tax return or pay any taxes in the U.S. for the intrests he gained even if he declares the intrests in Italy and pays taxes ?
Appreciate your help and answer.

Since the interest is earned from a U.S. source, taxes are due at a 15% rate per Table 1 of IRS Pub 901.

The taxes would probably be withheld by the financial company at the 30% default withholding rate unless you file Form W-8BEN with the company claiming the lower 15% rate.

If the taxes are withheld at 30% and the tax year ends before an adjustment can be made to the lower rate, the excess withholding can be refunded to you by filing Form 1040NR.

If the adjustment is made in time and the withholding is done at 15%, no refund would be due and thus no tax return would be required.

You should be able to claim a credit for the payment of the U.S. taxes on your Italian tax return.