Hi,
No, for price difference it's allowed (and recommended) to have 30/3. Check if note 2041451 -
System issues error message FI149 when you post a goods receipt is implemented in your system (or if it's relevant). If it does not help, I would have debugged to see on which commitment item and why the system issues the message. If this is not an option for you, you'd better open OSS note, as analysing this problem remotely is tricky.
Regards,
Eli
P.S. Also, consult note 2044939 - MIGO: Error FI149 'Posting not possible; incorrect financial transactions of the cmmt item'