Specified quantity is greater than the open quantity

What causes this issue?

The delivery quantity of all batch split items is greater than the
original delivery quantity of the main item. The open quantity of the
main item is set to zero. The cumulated schedule line quantity of the
main item is equal to the sum of the delivery quanity of all batch
items.
If the delivery quantity of a split item is reduced, the main item
contains an open quantity again. Since processing is carried out per
line on the batch split screen, the items whose quantity was reduced
are to be entered first.

How to fix this error?

To set the open quantity back to zero, or to reduce it, you can reduce
the delivery quantity of the main item respectively. To avoid the
effect on the open delivery quantity described above, change the items
whose quantities are reduced before the items whose quantities are
increased.

Error message extract from SAP system. Copyright SAP SE.