How can I enter the exact amounts on both sides of a foreign currency transfer?

To enter the exact currency value for both sides of a foreign currency transfer, use the exchange rate field's ability to interpret math expressions.

In the example screenshot there are two accounts: Checking in USD and Savings in GBP.

When making a transfer from Checking to Saving and you know the amount was 150USD in Checking and 90GBP, but you don't know the exchange rate, enter the transaction in checking as shown in the screenshot.

The transaction is entered in the Checking account with the category set to the Savings account. The payment field is set to 150. To make sure that the other side of this transfer is exactly £90 just enter the rate as a fraction foreign_amount/local_amount (in this case 90/150) and press the TAB key to have moneydance compute the appropriate exchange rate.