Bad service at Spec savers west street durban

My mother went to Spec savers west street Durban for an eye test and glasses, she specifically told the lady at reception that everything must be done within the available funds as she is not in a position to pay extra of which the lady confirmed the funds so received the glasses, weeks later she received a call saying she owes them over R2000 because the medical aid short paid, this is disturbing because she did indicate that if there are no funds available she is willing to wait for the availability of funds.