Being forced to keep airtime I do not need.

I bought 1000 airtime on my bank app by mistake and I wanted 100 airtime, immediately upon realising this I called mtn to refund me they informed me to send an email to their customer service would call me back, they never did I called them again they asked me to call on a Monday I did but they refused to refund me I need this money for traveling back to work after the lock down who can assist me in getting this resolved, because I have no other means of traveling back and I really do not need a 1000 airtime.