Vodacom is ******** people with their contract

I have a contract with vodacom which will end next year Aug I take a Sim data contract which the promise to give 80gb 50 gb day + 30 gb night I notice that the data they give it 50gb only 30gb day and 20 gb night and I have a contract that I have signed when I go to mega city branch in Umlazi the say they can't assist me they never have such contract and the sales person who assist me is no longer working for vodacom of which is not my faulty and I wanted to cancel the contract and they told me to pay 5600 but the rob me with data all this month vodacom is really ******** people and when you complain they don't assist