Refusal to refund

I called Armstrong's last Monday and asked them to keep a certain book, I then went t buy it on Wednesday the book is second hand by the way. On Saturday I received the same book from Unisa I didn't know that I would get the book. I discarded my proof of payment because I didn't know that I'd have to return the book. Now Armstrong's is refusing to take the book back and refund me, On top of that they say the only way is to buy it back from me as second hand! I mean it hasn't even been a week, I bought the book there, I don' need it anymore, I am not a ********. Why can't they just take back the book?! Perhaps someone else needs... Why must I sell a book I haven't even used as second hand?<br> <br> This is really exasperating!