Apple India — lightning wire not replaced within warranty

Address: 560068

I approached imagine, apple authorised service center, for the replacement of my defective charging cable. At the first instance they have accepted it stating that it will be replaced within 2-3 days. While accepting it they checked it throughly and confirmed about the replacement. Later they rejected it sataing that warranty is void due to discolouring of pin. This they have not informed while accepting the charging cable. My question to the company is why have not checked it while taking it in their coustody and informed me that warranty might be void due to discolouring. Secondly there is a possibility that discolouring of the pin might have happened in their coustody.

Request you to assist in this regard.
Thanks.