Configure reports in Vtiger CRM

I have Vtiger CRM and I am trying to create commission reports. I want to be able to create a report that will show all customers grouped by dealer and calculate commission due for each on a monthly basis. I have a field a filed which is formatted currency euros called commission due. If I enter say 5.00 in there is then next time you open it say 5.06 or all sorts of crazy values. This is a small project with a small budget.