Thank you for your reply.
The Foreign Principal will not pay the IGST.
Agent's Invoice to the Foreign Principal will be for USD 100 and Foreign Principal will remit only USD 100.
So cannot the following apply?
Where the value of supply is inclusive of integrated tax or, as the
case may be, central tax, State tax, Union Territory tax, the tax amount shall be determined in the following manner, namely,-
Tax amount = (Value inclusive of taxes X tax rate in % of IGST or, as the
case may be, CGST, SGST or UTGST) ÷ (100+ sum of tax rates, as applicable, in %)