Immigration law addresses whether it is legal to make a payment to a foreign national for services performed within the U.S. This determination is based on visa classification and type of payment involved.
Tax law address whether taxes must be withheld on the payment and whether the payment must be reported on Form 1042-S.
As a general rule, Calvin just pay 30% in U.S. federal taxes on payments to foreign nationals for services performed in the U.S.
Additional IRS information is available using the following publications: