ok, I think this may be possible, however it's a fairly horrible solution.
1 - load from your qvd, and include an additional field, a concatenation of Case_id and Amount.
2 - use a concat to concatenate a comma separated list of all possible values of the new concatenated field
3 - using the concat string from step 2, query your database for any records where the concatenation of Case_id and Amount isn't in your list.
4 - Concatenate on to this table from your existing qvd 'where not exists(RowID)'
As I said, it's horrible.
A better solution would be to add a new modified date field to your source table, and set up a trigger to populate it on insert or update.