Upfront Deposit question

[yoon]

I received payment from a customer before work was started. Lets say he paid me $1000, I used $900, how do I record this and show that my profit is $100?

I tried following what Intuit suggested (http://support.quickbooks.intuit.com/support/Pages/InProductHelp/Core/QB2K4/core2/retainers/task_recording_retainer.html) but, I end up with a negative invoice which QB doesn't allow.

[Joe Williams]

You first create an Other Current Liability account named "Customer Deposits" and an item Cust. Deposit, of type Other Charge.
Enter a Sales receipt for the customer using the Cust. Deposit item for $1000.
When you do the work, enter an invoice with items for the work performed and the amount per item. The last item on the invoice is the Cust. Deposit for a -$1000.00 to apply the pre-payment to the invoice.
NOTE: the work item amounts should total to $100.00 or more and is it is more then the invoice will have a balance owed by the customer.