This noon I had a chat with store manager (we discussed same issue before), he said only the rebate printed out is valid; at least, that was instruction given to him. I asked about the leather chair, which was not only free, but also put money back to the pocket. He explained, when it is house brand item, OM usually honors rebates -- for example, Microadvantage DVD writer, sold only through OM; but not Khypermedia, OM has no control over Khypermedia's rebates. It will be up to Khypermedia to decide.
I probably will keep it, after all, not a bad price.