Hi Wouter,
Thanks for your update.
We regret to inform that it is not possible to process the mentioned validation (checking with cases) within our source to display the error message, and also as per the standard - the database fields need to be mapped correctly with proper Scheduler fields name (as per the casing standards). Moving such kind of validation for each record at source side will lead to the lack of performance.
Please let us know, if you need any further assistance.
Regards,
Karthigeyan