My accountant told me that payments to foreign persons or companies are subject to 30% federal withholding unless:
If I'm understanding the situation correctly.
Assuming you are working as an independent contractor then the US company will just pay your invoice. Your US client would not pay US taxes or money on that $750 they paid you. Depending on your country you will have to pay taxes on that 750 received and maybe should be charging a service/sales tax on some items. But, you US client should just be paying the $750.