As per standard process on Work Order Backflush Booking is warning you that you try to receipt more than the planned quantity.
It's very good that the system is warning you but you have no option to cancel to the exceeded booking.
The below script will help you to overcome this situation, it will display the warning message and block the posting
Paste below script on "fert_retrograd_abbuchen.src" project file
In case the user try to book more than planned quantity the system asks him "Check the Planned QTY" and blocks the program for continuing.
If you want a better warning text you can write after "Messagebox"
if <total_qty> n> <dw_1.item.sollmenge.value> then
Messagebox= Check the Planned Qty