If one has problems with the creation of Discount recalculations under Invoice > General > Discount, the in this article described case is a likely reason.

It is not possible to carry out a discount correction when an In Advance invoice is available but a Final invoice has not yet been created.

However, the deduction of the special discount is in this case already shown in the 'Bookings' tab of the suborder data record (MG1 -> MN1).

If you cancel the In Advance invoice, the problem should solve itself.