I've never attempted a post-purchase PM at BB before so I don' t know exactly what BB does; however, this much I do know:
I once purchased stuff at BB (containing BB MIR offers -- rebates sent to Calais, ME) which I subsequently found significantly cheaper at a no-name web-site. Before I received my items from the no-name web-site, I sent in my BB MIRs. When the items arrived from the no-name web-site, I took them to my local BB (w/ my BB receipt) and returned them. The refund went through without a hitch.
A few weeks later I received rebate rejection letters from the BB Rebate Center claiming (in each letter) that the item I purchased had been RETURNED. Since this was indeed true, I couldn't dispute the claim.
My point? If/when you do a BB PM, if they do it in such a way that the ORIGINAL PURCHASE GETS REFUNDED followed by a NEW PURCHASE AT THE PM'd amount, then mailing in your rebate form w/ your ORIGINAL REBATE RECEIPT may cause your MIR to be rejected! If anything, when you get issued a new PM "YELLOW RECEIPT," if you don't receive a "YELLOW REBATE RECEIPT" from the PM, I highly recommend you send in a XEROX of the "YELLOW RECEIPT" w/ whatever REBATE FORM you submit.
Just my 2¢ worth.