EWM posting change with a EWM-WCU connected system leads to stock balance discrepancies

[Mathew_S72]

Hello EWM experts,

in our scenario, we use a EWM-WCU connected storage type (like described here from @Gunter ) to connect an external warehouse management system (WMS) in "black box" setup (as described here)

"Black box" means that external WMS is full blown and does the complete inventory management on quant level, meaning WMS keeps all SAP EWM stock identifying fields (like /SCWM/DE_MATNR, /SCWM/DE_CHARG or /LIMEB/STOCK_CATEGORY,...)

Now different issues arise depending on the initiating EWM process for the posting change (PC)

1. /SCWM/MON > Stock and bin > Physical stock
Select stock on a storage type which is linked to external WMS
Execute Action "Change Stock Type" and change stock type e.g. from "F2" to "B6"

a) leave WPT empty, post
result: stock type is changed in EWM immediately
one confirmed WT with 2 items written (TRART=7) to document the posting change
1st item reflects the old stock data, 2nd item the new stock data
No WHO, no IDOC to external WMS ---> stock balance discrepancy created by this posting

b) provide a WPT
result: stock type is still changed in EWM immediately
one confirmed WT with 2 items written (TRART=7) to document the posting change
1st item reflects the old stock data, 2nd item the new stock data
a open WT + WHO + IDOC is created additionally, containing just ONE item
this item contains the new stock data only

--> /SCWM/MON method coding comments said something like AdHoc postings on physical stock are always treated as "goods movement before WT". All these kinds of PCs either create an instant stock balance discrepancy bewtween EWM and WMS and/or are "useless" because IDOC informing the WMS about the stock to be "updated" is "incomplete" as IDOC must contain source AND destination data.
Additionally, the GM in EWM is always done before WT creation, WT cannot be confirmed with differences if WMS was not able to update its stock

2. Use /SCWM/POST, select stock on a storage type which is connected to WMS
a) create PC immediately -> same behaviour as ad hoc PC from monitor
b) create WR (SPC/TWPR), create WT for it in /SCWM/IM_PC
WT + WHO + IDOC is created just containing ONE item
This item contains only the source data. Again, this IDOC is "incomplete" as destination data is missing to allow the WMS doing the similar stock update

Long story short, in ERP WM there was an interface storage type 922
If you did a PC in ERP IM or in WM, you got a WM transfer order with 2 items
1. storage bin -> 922 (old stock data)
2. 922 -> storage bin (new stock data)
The WMS uses 1st item values to select its stock and the 2nd item values to update it

This seems currently not possible in EWM ? Any experience with this topic ?
We currently consider to extend the /SCWM/WMTOID01 IDOC to add missing data to get similar behaviour as known from ERP WM

Best regards,
 Matthias
... and yes, we use use lot of EWM-MFS driven warehouses, but this is not an option here