Unable to enter negative contract line on Project Contract

Summary:

We are unable to enter a negative contract line while creating a project contract which seriously impairs our ability to liquidate the advance payments made by customer and tie the contract back to the customer order.

Content (required):

We are in defense manufacturing. We bill the advance payment initially on project contract using project billing through to AR.

Then after item is shipped, we do the billing for that item using another contract line. At the same time, we liquidate a portion of the advance in the proportion of the billing amount using a liquidation line which is negative amount.