Manager "Roey" rude and unhelpful

I went to McDonald's Eastgate on Friday 08 Nov at 18:30. Upon arriving there I asked to order a meal from the McDonalds App which I have on my phone. I was asked by the cashier to Redeem the offer and scan the bar code, which I did in from of her. The system did not register and did not ring up the offer. I was asked to scan the offer again, however the redeemed offer has already expired due to the time-out. I asked the Manager by the name of "Roey" who was standing next to the cashier what to do now. He insisted that i did not redeem it and imp**** that I am lying or that I have used it before that day. I advised him that him system is faulty and that it should not be my problem and he has to overwrite the transaction in order for me to get the promotional item. He kept on insisting that I am lying and refused to help me. I am extremely unhappy about the service received from the manager and that I am being told that I am lying. I am a regular customer at the branch and has never received such poor service!!!