Accounting for Exchange gain or Loss

We have a client in NZ, both our company operated under the same contract in the past.(lets say company X and Y)

We issue our invoices in USD. Once it comes to payment. The client pays the GST portion of the invoice in NZD dollars (exchange rate based on the current day) (NZ country allowance by legislation), the rest it’s paid in USD. From the client perspective the invoice gets paid in full.

It gets complicated once the money received in our bank account as per "X" has it’s own bank accounts(only USD) but not NZD one. So the GST portion of the "X" Invoice gets paid to a "Y" bank account, then internally gets transferred from "Y"to "X"by CIT. The NZD fund received gets converted to USD using the monthly "Y" rates. As a result when it comes to allocating the payment in Oracle we have less or more money depends on the rate used at the time of conversion.