Hello-
In 2009 RTC, there is no way to do an Item Charge Assignment on Purchase Returns. As a matter of fact, there is not even a "line" option when you click on the Action button (lightning bolt). So, of course when you try to post the return with the Item Charge, it won't let you because it is not assigned. The classic works fine and Sales Returns in RTC also work fine. It just seems to be Purchase Returns in RTC.
Also, it is working in 2013 so I'm assuming there was a fix. As usual, I can't find anything about it in the Knowledgebase. If anyone knows anything about it, I would appreciate the help :)
Thanks!