Wrong Cost in GRN

If you have stock in some item with the wrong cost, and already sold some of the item, refer to the example below to have a better understanding on how to correct it.

Example:
2009-01-02: stock in item and create GRN with cost RM13
2009-01-03: create invoice and sell the item
2009-01-05: realize the cost of the item is only RM10

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