Geyser Safety Valve Refund

As per the quotation of a plumber I bought a geyser safety valve at Cashbuild Krugersdorp. Unfortunately, the plumber had misdiagnosed as no valve was needed. So 2 days later on 14 Dec 19 I tried to return it and get a refund but was refused. The reason being that RCS credit cannot be refunded unless you exchange for something else. I don't need anything else but a refund as part is still in original packaging. Please help me get fair treatment.