Changing Charge Calculations from SAP TM

Hi experts.

Im trying to change the cost of the freight order using the BADI /SCMTMS/TCC_RULES on the method CHARGE_CALC_THROUGH_FORMULA but Im having some troubles.

I tried to change the AMOUNT property from the CHANGING Structure but it didn't save my changes. After I tried to use method MODIFY from the Transaction Manager and changing the Business Object but when i try to call method SAVE I get a ASSERTION_FAILED dump.

Anyone could lend a hand?