Hi Mirco,
This should happen in the object itself. But when you update SAP will automatically apply its rules and calculations (discount calculation, taxes, etc...). That may in fact change some values in order to maintain the document integrity.
However this should be reproducible in the client itself, eg.:
Setting quantity one and the total to half the unit price, will result in 50% discount.
Can you give an example of the data in use, and the code in use?
This will help us help you.
Cheers.
Best regards,
Pedro Magueija