This Error is because of no valid business scenario
The error that you receive "Buyers CUM(945) higher than 'total cum. quantity delivered'(675)" is simply not a valid business scenario and hence the baan SCH schedule fails.
In practical business scenario , you can never have your buyer (Customer) CUMS (945) more than your internal CUMS(675) delivered qty and hence baan fails the SCH Schedule.
The Only valid scenario is your internal CUMS being equal or more than the Buyers (Customer) CUM because of some in-transit shipment.
So you will have ask your customer to check their CUMS calculation.