Does this happen on "Credit Memo" in AR Module?
During the setup about Transaction Type,you can define the <Creation Sign> as Negative / Positive or Any Sign.
then system will give the validation for the total amount of transaction based on this setup.
Please share the diagnostics of this transactions.
Thanks Micheal. It seemed to be problem at the user side, user just responded as to she placed a placed order and it worked fine.
Thanks for the tip though.