Coke a Cola returns

I purchase my 1.5 Litre Coke a cola on a regular basis at Spar Montagu,so I accumalate the bottles and then cash it in and spend the money in Spar Montagu.Today they told me that for every bottle that I returned,I must buy another Coke for.Spar Montagu you get returns on the bottles that Coke a Cola collects on from you,who gives you the right to change the rules.You have lost another customer that will not buy in your store again.