Repair centre can't repair.

I upgraded my contract on the 27th of March and received the S6. In the following week it started to give me problems and the screen went blue. As it was a long weekend and I was on leave I could not return the phone to the shop within the 7 days. It was returned on the 7th of April which is technically still within 7 working days and as such I feel should have been an out of box exchange.<br> The phone was then sent in for repairs and was returned to me on the 28th of May. The phone was working but as soon as I plugged it in to charge it told me that the charging was suspended as the battery temperature was too low. I took it straight back to the shop the following morning and it was again sent in for repairs. Now I have been told that I must pay R3752.45 to have it repaired due to corrosion damage.<br> My frustration is how did it get corrosion, your repair team has had the phone for the last 2 months and when it was returned to me on the 28th of May they had replaced the battery and done a software upgrade, why did they not see any corrosion then??? I returned it immediately on the 29th and now suddenly they see the corrosion, what kind of service is this???