Hi,
You may check the user exit: EXIT_SAPLMBMB_001, or MB_CF001 or BAdI: BADI 'MB_DOCUMENT_UPDATE' or MB_MIGO_BADI or similar.
Give logic in such a way that the system has to pull the exact valuation price to the "amount in LC" field based on the valuation type. The value for valuation type will be available in MBEW table. Make the field "amount in LC" display only for MB1B for 312 movement. If the correct value is entered, it wont trigger AUM transaction key.
Regards,
AKPT