You can file a Dispute against another user you are in currently working in a Gig or Contest with.
To lodge a dispute over a payment, go into the studio of the Gig/Contest you are working on and select the "Payment" tab. Select the "Negotiate" link. You then need to describe your issue in detail.
Finally, you should enter the amount you are prepared to pay for the project (if an Employer) or wish to receive for the project (if a Employee). The amount could be between 0 and the total amount of Gig/Contest in question.
The other party has a number of days (4 if the other party is a Employee, 7 if the other party is an Employer) to respond to the claim made by the complainant. If the party does not respond within the given days, the Dispute closes in your favour.