Promotion

I am using SuitePromotion and the sales order quantity  is less than the ship & billed quantity.  The SuitePromotion calculate the promotion on the sales order level, and it applied the same amount on the invoice level.  For example, say item ABC is a flat discount of $1 each, ordering 4 case would calculate a $4 discount.  This $4 discount is applied toward the invoices, regardless if I ship 1 to 4 cases.  This is ridiculous, if I only ship 1 quantities, the $4 discount still applies.  It should calculate $1 discount for 1 quantities sold.  Are their any solution or workaround to this problem?  Thank you.