Shoes

I bought slip on shoes around november it was 2 pairs black and white now the shoes are broken underneath and they only willing to give a voucher for 1 shoes which is the black one because the black one is tearing on the outside and the glue from the shoe is sticking out. I fail to understand why they only want to return one shoe. Spoke to the person who is in charge of the quality team she said there is nothing they can do.