Hi gosiana,
Negotiated or spot rate on a currency transaction with a third-party is seldom correct in producing the expected conversion to the exact penny when you recalculate after the deal has been agreed and settled. Furthermore, QBs exchange rate box can hold up to 6 digits after the decimal.
I suggest you always work out the exchange rate on a calculator, round up or down to 6 decimal points, and use that in the Funds Transfer form on QBs. Of course you may have to wait until you have all the paperwork to find out the exact amount paid in GBP or Home currency for the exact amount of foreign currency credited to your account.
Very occasionally you may encounter a cent rounding up or down difference in the currency figure, but that's another issue that requires a workaround.