SAP Message AJ619 - Document and contract have different currencies; conversion not possible

Diagnosis
This message involves a problem with the currency conversion to theeuro.
You tried to reverse an existing document with the manual billingtransaction, but the currencies in the documents are different: Thecurrent document has currency &v1&, while the existing document hascurrency &v2&.
To convert correctly, the price used must be defined in bothcurrencies. The price for both currencies must be identical in both thetime slices and the blocks.
Please note the following when converting the price and net amounts:
The net amount is converted to the euro using the valid conversionrate. The price amount is taken from the price table. This results inthe following problem:
Normally, the net amount is calculated by multiplying the quantity bythe price. Because the price amount from the price table can bedifferent due to the mathematical conversion to the euro, multiplyingthe quantity times the price amount does not necessarily result in thedisplayed net amount.
Example:
Old document in DM:
Price amount in DM ,,1.00 / kWh
Quantity,,100 kWh
Net amount in DM ,,100.00
Price in EURO,,0.5*
*The exact price in EURO is 0.51129...
Before the transfer to the new document, the net amount is multipliedby the defined conversion rate. The new price amount is taken from thetable.
New document in EURO:
Price amount in EURO ,,0.5 / kWh
Quantity ,,100 kWh
Net amount in EURO,,51.13 EURO
The net amount differs due to rounding the EURO price beforemultiplying the price by the quantity.
A simple multiplication would result in a net amount of 50.00 EURO.