subtract (decrease) order quantity

i search the forum and i can not find this solution.

i have existing order (ex: product A with 24 qty order) and i want to decrease the order quantity to 4.
it happen to me due to defect item, and i had to inform the customer we only can ship 4 and customer agree with it.
i go to order > edit > select item and input -20, it didn't work.
i search this forum, i can not able to find solution and also extension is not there.
i don't want to tell the customer that reorder with new quantity.

is there anybody who know how to solve this problem?

Please help~~~ me~~~

Version? Did you try "4" rather than "-20"?

Version 1.5.6.1.
if i try 4 and add, it just add the quantity.

here is the screenshot.

Where you started with quantity 24 and wanted 4 instead, I meant to insert 4 by itself and save, rather than to insert -20 and try to add it. The Add button is to add products, not to sum (+/-) quantities. Just overwrite the 24 with the 4, Save the 4, do not "Add" anything to do that. In the image, different numbers but same gameplan -- Add does not do arithmetic, it adds products. That seems to be where you became confused, you're too close to it.