Airtime disappeared

I completed my 24 month contract with MTN approximately 6 month ago, and have just continued to pay my monthly account which gives me R350 airtime. On Friday 14 August I decided to cancel my contract and find another supplier as the MTN signal is very weak in my area. At the dealer I was asked when I wished to cancel and I said as soon as possible. He informed me that my last payment would be at the end of August and from there on it would be a pay as you go phone. I sigmed the papers and left. Two days later I had enough signal to make a call, only to be told that I had insufficient funds. This is impossible as I still had R750.00 airtime on my phone. I returned to the dealer who said that as I had cancelled my contract my airtime was lost even though the last payment had not yet been made. Is this not theft? Surely if I have to still make a further payment at month end I would be entitled to have my paid up airtime available until the last day.