Store refusing to refund for a matress

Bed delivered last Friday. I ordered a King extra length but the mattress is too big for my bed frame. I was told my the delivery guys that the procedure is to leave the bed until they bring a replacement. I then called the sales person who said he will work out the price difference for the sizes of the mattress and that he will also arrange for an exchange to a standard size mattress instead of extra length. There was no request for me to send the mattress back while the warehouse guys were in my house. I have now been contacted by the store manager to tell me that he will not give me the price difference between the two and that me changing my order is costing him money. I want a refund of my money. The mattress hasn't been used and was left in my house on the recommendation of Dial a bed. Had i been told by the warehouse or the sales person on Friday to return the bed immediately or they would be unexplained cost I would have told the warehouse to take the bed.