Hi,
Normally in project sales (construction contracts for turnkey projects etc) sales order line items are sold as non stock items or just like services. Make sure that the WBS element is assigned as a cost object in the Sales Order. This will ensure that the total of sales order value is copied as plan revenue in the WBS element and the portion that is actually billed (by mile stone billing) will be posted as actual revenue,
Regards,
R C Sabat