This exact same thing has happened to me. I used my rewards and they cancelled my order the next day. My rewards were never returned and my balance still says $0.00 three weeks later. Phoning Customer Care, they just tell me they have no record and I need to phone Sales. When I phone Sales they tell me to phone Customer Care. 11/27 = They appear to be back onto my account. Thanks again for your assistance. 
For the second time cash in my Dell Rewards with an order.  Dell cancel my orders without giving any reason.  According to Dell the Reward is already spend even the orders cancelled.  Calling Dell support just get transfer to different department.  First CSR said the verification depart had hold on the order.  Verification depart said there is nothing wrong.  Transfer me to sale department then order support department.  The first time it happen they said they can not refund my Dell Reward because I spend it on the cancel order.  It took over three weeks for a helpful CSR got my reward credit back.  For the second time Dell has cancel my order without giving me any reason again.  Same Reward is stuck in the bait and order cancel game.  Oh! funny things are CSR could never find my information nor the cancel order.  Even I sign in to place order, looks like every time I order I get a new customer number.  
Dell cancel my orders when I use Dell Reward