am forced to pay for something that i dont know.

During the month of May or june but it was when MTN on strike i went and did an upgrade ...at protea Gandency Mall and according to me my upgrade was successful course the provided me with a new phone. mean while the person who was assisting me did not close the bill on my contract course am on top-up...now i have a bill that i need to pay which is it was not my fault that whoever that was assisting me did not do they job properly.. i called mtn and told them to credit the invoice and re-invoice me my correctly, course they can not hold me liable for their negligent. do you job right,....