There is a 15% tax included in the price. Usually, Sage calculates and adds tax, but in this particular scenario, we need to deduct it. So, when the supplier sends an invoice for 115, we should retain 15% and pay the supplier 100.

The 15% withheld will be paid to the government later.

We're currently exploring the optimal method for handling this situation.

I think possibly we're overcomplicating this. why not just do an invoice for a 115 (you can setup the tax to record in a separate account, then just make the first payment to the supplier for 100, and then do a second payment for 15 and send the that portion of the payment to the government instead of to the supplier. 

I think possibly we're overcomplicating this. why not just do an invoice for a 115 (you can setup the tax to record in a separate account, then just make the first payment to the supplier for 100, and then do a second payment for 15 and send the that portion of the payment to the government instead of to the supplier. 

This.  I wanted to write the same thing but thought I must be missing something obvious.  Especially if the invoice is from a foreign supplier, why need to think of the tax at all, unless there is some need to record the tax for local government reasons?.  As you said just raise an invoice for $115, remit $100 to the supplier, $15 to the Government tax authority.  If your Sage 300 system generates payment files for cheques/checks based on the supplier card, just make these payments manually.  I figured it was an overseas supplier as it doesn't sound like something iamasp27  has to do regularly.