Losing money paid for airtime

I have been dealing with MTN to upgrade from iPhone 5 to 6, but after waiting for a while for stock I finally able to upgrade but they can't accommodate me on my current contract i.e. Topup350 the only have 350 anytime but any new contract I choose I loose my money accumulated i.e. R1400, the woman I was dealing with was very nice and addressed it with her superiors but no joy. I have been attempting to finishing the cash but I feel this is very unfair, this money can't be transferred, this is airtime I have paid for (on my contract) but am not able to keep!, How is this fair??