Hi Barb,
This error was reported to us a short time ago. We will soon be releasing a service pack for Order Entry 8.1D that among other things will fix this problem. The problem is exactly as you explained, that the Extended Cost was not being correctly calculated for Credit Notes when doing partial returns. This started happening in the Nov 2010 release of OrderEntry.
I don't have a specific release date for you, but it is reasonably imminent.
If you need this fix right away, a hot fix is currently available for OE that will address this problem. You can get this through your Adagio consultant. The upcoming service pack will contain the changes in the OE hot fix, plus more.
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Regards,
Softrak Tech Support