bug fixing - open to bidding

Hi, I have a problem with a web application that was designed in the Grails as a web portal for an Android application (running on Tomcat 7). There is a security bug that I would like to have fixed. When someone registers to the application, the same username and password can be used to enter and modify the database through the portal. Which is not requested behaviour. Only admin should have access to the database. The application was design by a freelancer that does not want to admit his mistake and I am looking for someone else to fix it. Can you please send me a quotation how much would it be to fix this issue? I am sure it will not be difficult. How much would it be to fix this bug? Thank you, Jan [login to view URL]@[login to view URL]