Since year 2005 there is limitation on table 5902 Service line which prevents entering negative quantities. It is on fields
Quantity
Quantity (Base)
Qty. to Invoice
Qty. to Invoice (Base)
Qty. to Ship
Qty. to Ship (Base)
Qty. to Consume
Qty. to Consume (Base)

The code in OnValidate of this fields is

if Quantity < 0 then
FieldError(Quantity, Text029);

We are removing this limitation for last 10 years and the negative quantities are working as expected, e.g. when you are returning old item and shipping new one through one service order. Rest of the code is ready for cases that quantities could be negative. Only limitation is this test, which makes the whole service documents not in line with rest of the system, where negative quantities are working as expected.

Now, when using extensions, is this limitation breaking usability of the module and there is no easy way how to overcome this.

The idea is to remove this tests or make them possible to handle through handled pattern, making the Service module more in line with rest of the system.