Turbo Refund not honoured

On behalf of Tora Concrete, we have been in business with the above mentioned organization and in June 2017, we purchased a TURBO CHARGER for a truck after we fit it, it covers 132 kilos and fails, I called the sales representative informing him of its malfunctioning and was told to bring it back with its original invoice of which I did. I was then given a form to fill all necessary details as a result. by early July all submission was made in a twist tail, I was told that the refund could only result after purchasing another TURBO of which I did. Until up to date, I had two calls, one from sales representative an another one from the manager I suppose, in last two weeks I haven't heard anything and have not received my REFUND. Can I have an urgent respond as this has been dragging for months and its affecting my business. the company name in full is "Mercedes Benz Commercial Vehicle Centurion".