Vodacom trickery

On about 21 June I tried to cancel a contract with Vodacom that had already passed its renewal date. MTN was offering a "Made for me" package with an iPhone 7. Vodacom volunteered to match the offer, and also included a further 2gig data thereon. The deal was OK'ed by the commercial department after about 4 days. Stock had to be found but as I was away till 16 July I was not perturbed. On my return no stock had been secured and Vodacom has tried to get me to change to an iPhone 8 at an additional R150/month = R3600. I have wasted numerous hours on the phone to resolve this, and run the risk of missing the deal with MTN as well now. Why on earth volunteer to match a deal if you do not have the ability to do so? Is it "false advertising" or a simply ploy to sell a bigger contract to me?