Refusal to undress mannequin to supply customer

Today I went to Markham to buy a suit unfortunately they did not have the size I was looking for in the blazer. They even phoned around to other Markham stores in East London to look for the size I was looking for to no avail. No Markham in East London has the size so as I walked out disappointed I noticed the mannequin in the window was wearing the blazer with the exact size I needed so I rushed in and asked them if they could change the jacket the mannequin had on and let me have the one it had on. The store manager said that they were instructed by head office not to change the sizes the mannequins were wearing until the promotion for the attire it was wearing expired. I begged them just to make a simple swap to the sizes but the refused on instructions from above as they claimed. I left empty handed and disappointed and I so want that suit for this very important event I'll be attending. What rule is that really now? The mannequin is advertising a size you're out of stock with and yet you refuse to give the customer the last one you have available.