Not able to change Quantity and Request Date on Order Line

Hi we have two cases at hand:

1. Problem #1

 Order Type : Standard Order

Line Status of Model Line : Awaiting Fulfillment , Configured Item : Supply Eligiblle

Action Performing : When trying to change to change the Quantity from 2 to 1, it gives error "Model Price and Ea not on Price List".

2. Problem #2

Order Type : Ship Direct Order

Line Status of Model Line : Awaiting Fulfillment , Configured Item : Awaiting Reciept

Action performing :When trying to change the request date from 30-Mar-2012 to 29-Mar-2012, it gives error "Model Price and Ea not on Price List"

Could someone please suggest the cause of this error and how to resolve.