The situation which you have quoted has a very simple solution.
Ask the client to have a track of the orders against which advances have been received from the customer. Then when the Sales invoice is raised against that order the netting off can be done.
17 June 2010
This situation is arise when you have recevied agaisnt separate sale order which is to be executed after Balance Sheet date. This is reason that your client want to shown it separte. There is no problem to shown it separate in Balance sheet if it as per above situation.