Hello fellow experts,
I have researched the SCN for this issue, but unfortunatelly have not found a similar case.
In our implementation, we have created a manual discount for our samples processes, condition type ZTPA. We have customized it to be both header condition and item condition, and we have customized KMANU (Manual entries) to be "B, Automatic entry has priority" as we would like to have both MD for the generality, and manual for exemptions.
The way SAP's help explains KMANU "B: The automatic entry has priority. If a condition record exists, the condition cannot be entered manually" seems as we could manage the condition type manually regardless if it is header or item, however, in trying to process the condition manually in a sales order, the message V1213 is triggered stating that condition is not allowed at header level. 
What's even more interesting is that when we attempt to enter the condition at item level, the action is successful and condition is set manually.
I have tried to debug the issue, but so far has been unfruitful. Has anyone faced this issue before and can save us some time in OSS note?
Thanks to all in advance!
Nicolás Posada