The summed-up lines in picking list must be smaller than or equal to the ENTIRE UNDISPATCHED quantity from this line of order. That is, regardless whether the order was dispatched from some other warehouse using a picking list or not. It can happen that order is being displatched from two separate warehouses, where one of them is not related to any picking list (see picking list and changing quantities in order).
If you want to decrease the quantity for dispatch below the total undispatched quantity, the following error is returned:

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Solution:

You cannot decrease the quantity in picking list because issue is already confirmed! Such a change is possible only if you first change quantities in the invoice issued!