tax after credit memo

the scenario is the following,  the functional currency is MXN,  so we created and Invoice, applied a credit memo and generate a receipt for the missing amount, all transacction are in USD

1.Create an invoice    the exchange rate is 13.8667 MXN  for 3450 usd, tax line is 450usd, and 3000usd revenue line.

2.Create a credit memo and applied, the exchange rate for the credit memo is 13.1067 MXN, the credit memo is for 1725 usd, 1500 usdcorrespont to the revenue and 225 usd for tax linea, the is and loss between the exchange rate for the invoice and the exchange rate for the credit memo, the difference is 0.76 Mxn so, the line for Gain is 1311 MXN.