We were promised a refund and didn't get one

I posted previously about Kwikhot's gravity fed solar geyser which does not work in winter, even if the sun is shining. After some discussion we were told to contact Paul, the sales manager in Cape Town. He agreed that we would get a refund as the product was misrepresented ...i.e. there was no information to say that it does not work when the ambient temperature is below 20 degrees (according to one of their technicians) even when the sun is shining. He told us we would be refunded on the 15th of the month by Jo'burg head office as that was when they did their account payments. We weren't refunded. When we enquired he said he was sorry he forgot to inform them of the refund. He said we would be paid from the Cape Town office. When we weren't refunded the following month we asked why and he said he needed the invoice to prove we had bought the geyser. (it had been returned to their Cape Town depot months ago) . We sent the invoice. Still no refund. When we asked when we would be paid, he then informed us that they couldn't refund it because it had been used. It was on our roof for a week, then we removed it. I think that had been clear from the outset, and to keep us waiting for almost three months and then to tell us he wouldn't do what he had said he would do is not acceptable. Now he says he will try to sell it for us, and although that sounds good, it hasn't happened yet.