HU is assigned to inbound/outbound delivery. Differences are not allowed

What causes this issue?

With this transfer order, you wish to move a handling unit that is
already assigned to an inbound or outbound delivery. Entering a
difference quantity in this case is not possible.

System Response

The entry of a difference quantity is denied by the system.

How to fix this error?

There are two different possibilities:
The handling unit is assigned to an inbound delivery
Confirm the transfer order without specifying a difference quantity.
Afterwards, post the goods receipt for the inbound delivery. Then you
ca n correct the stock of the handling unit (either during a physical
inventory run or by creating a manual transfer order).
The handling unit is assigned to an outbound delivery.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.