Tata Boksburg not refunding car parts in perfect order

I firstly ordered a sump and an oil cap for my TATA Telcoline. I collected the parts from Tata East Rand a day later. I then ordered a oil return pipe and an oil seperator for the same vehicle a week later. In the interim we found that the diesel pump and the head of the bakkie were uneconomic to repair and due to the cost to replace these two parts, it was better to get another engine for the vehicle. I took the unused parts, in perfect order, back to TATA Boksburg. I spoke to the parts manager and he said they couldn't refund because they were slow moving parts. He also said that there was a clause on the invoice that stated that specially ordered parts are not refundable. Which there is but you only are made aware of the clause once you have purchased the parts. The irony is that once the parts are purchased, you cannot return them! There is no sign at the parts desk stating the clause giving the client the information before he pays for the goods. The parts I ordered total up to R2000 and Tata Boksburg's attitude was that I just have to accept that I am sitting with parts that I have no use for what so ever.