Failure to honour guarantee of product

Morning. Towards end of February, I went to Midas at Heuweloord to buy parts for my Toyota RunX 1.4RS. Among the parts were a pair of front wheel shocks. On Saturday, 16 April 2016 at approximately 20:00, I took my car to TyreMart for a brake service. The technician realised that my left shock was leaking oil. He adviced that I should find my receipt and go to midas to claim another part because such kind of fault is covered by a guarantee. I went to Midas and explained to them.I was not happy with their response. A sales consultant advised that I take out the faulty shock, buy a new one and have it installed while they send the faulty one to have it assesed by Gabriel. I find his advise nonsensical. Why should I buy a part when I have already bought a part that is under guarantee. They fail to consider that that comes at an extra cost for me. I'd have to fork out R450 for a new shock and pay TyreMart R250 for labour. If they don't trust the TyreMart's technician that the shock is faulty, why can't they have it checked themselves or get their own technician to have a look at it. This is not fair at all to the me. I did not make the diagnosis, an accredited TyreMart technician did.