Vodacom is scarm

I went to upgrade my phone and was given one on that day, a contract was signed and admin fee was paid. 6 days later I received a call saying I must pay an upfront payment of R 8000 for the phone of which I was not told of on that day and is not on the contract I signed. The manager was rude to me and I felt bul**** by her, she did not even want me to ask questions. She is now threatening to blacklist the phone if I don’t pay the money. I call the costumer service and spoke to the supervisor (alome) but still did not get help.