Mtn

Hi I was order a phne on mtn online so her give me the wrong phone , so I sent it back I take the to nearste mtn shop and they help me to sent it back it was mei month now they said me invoices so why must I pay for something that I don't have , am phoning them every week they tell me they are busy with it 3months and they still can not give me a answer, today I go to the bank for a loan, but they can help me because mtn give me a bad record so what must I do