Selling faulty goods with no intention of assisting to exchange the goods

Somewhere in the middle- towards the end of September I have purchased a jeans from Sportscene as a birthday gift for my partner. In less than 4 months and wearing the item just about 5 times, the item has gone defective and faulty. Went today to go try and exchange the item for the exact same jean or an alternative based off the same value, whereby it has been rejected.. not based on not being able to, as I was able to, but due to the fact that there was a block placed on my account due to recent debt review added to my credit file. The store I went to today, N1 city, not helpful at all as they did not do anything to have the item returned or exchanged as said that customer services might be able to assist to unblock the account, retrieve the slip, and block the account again. They refuse to do that either. Now I as the consumer is stuck with an item that is not fit for purpose and not to the standards or quality that is expected from the brand of item purchased, and yet forced to pay for the item. That is not fair! Not by logic, not by standards and definitely not by consumer law! How can I be held accountable paying for an item they sold me, cannot be used, yet they not willing to assist or help in resolving what seems to be a simple matter, I am fuming!