UNKNOWN PAYMENT REQUEST

This people send me a letter that i owed lancet R1530 and i must pay it within 28 day, which i did currently they are sending me letters to settle another 227.53. I asked them for what, since I settled within the stipulated time frame, they didn't answer me, they kept on sending me to different people within the department, now they send me another letter that I owe them. I don't owe them since the amount they demanded was fully paid within the 28 days that they mentioned this is ****ing me off.