When you attach a sales agreement (for discounts) to a project you can only get a sales order that combines the price group on the project with the discounts, by starting from 'all sales orders' form in Sales and marketing and selecting the sales agreement. When you start from the project > new item task > sales order the field 'sales agreement' is blank and grey. 
You could also work from release from sales agreement but that is very user unfriendly for product category value commitment (first make header, then go to all sales orders, find your order, add the lines)