The way Dimensions determines if the difference in currency is an Exchange difference or a Rounding difference is based on the exchange rate. If the exchange rate is different on the two transactions, then any difference will be considered an exchange difference. If the exchange rate is the same, regardless of whether the currency values are changed, it will consider the difference to be a Rounding difference.
Examples below based using currency value of 150.00 and exchange rate of 1.5 (vat not shown for simplicity)
Example 1 - Change in Exchange Rate
Invoice
Home value = 100.00
Exchange rate = 1.5
Currency value = 150.00
Payment or Receipt
Home value 107.14
Exchange rate = 1.4
Currency value = 150.00
When allocated together, the Exchange rate is different so the 7.14 (107.14-100.00) will be posted to Difference on Exchange nominal account.
Example 2 - No change in Exchange Rate
Invoice
Home value = 100.00
Exchange rate = 1.5
Currency value = 150.00
Payment or Receipt
Home value 107.14 (value changed)
Exchange rate = 1.5
Currency value = 150.00
When allocated together, the Exchange rate is the same so the 7.14 (107.14-100.00) will be posted to the Rounding difference nominal account.
Posting a manual correction.
Sometimes you may need to post a manual correction to change the home value, without impacting the currency value on a customer or supplier. This can be achieved in two transactions.
Firstly, post a transaction of the same transaction type (example invoice) for the amount in home plus 0.01 you'd like to adjust by, and 0.01 in the currency value, for example:
I want to correct for an invoice that was incorrect by £1000 in home currency for a euro supplier. I would post an invoice with a value of -£1000.01 and -€0.01
Second, you need to adjust for the extra "penny" added, so you could post a transaction like this:
+£0.01 and +€0.01