Lancet wants more money

On the 19th February I had test done and paid the amount they gave me in full. Received a call on the19th April (2 months later!) about an outstanding amount of R500 and asked me if I know about this. How would I know if I paid the amount that they gave me and this is the first I hear about it? I have already claimed from the medical aid for these tests and cannot do so again. The fault is not mine. Whoever I spoke to said they will talk to their supervisor and get back to me. All I received in the meantime is the bill, no call back as promised. Till I get a satisfactory explanation why this tests now cost more this will not be paid and I will take this as far as I have to.