Won't reimburse me because of excessive stock

I made a R2000 purchase on 28th April 2015. Having discovered that 2 of the text books purchased were not the prescribed books, I chose to return these - value R670. Jonathan Ferreira refused to reimburse me for both books, stating that he had excessive stock of the one. I could only receive reimbur*****t for 1 book and had to take a voucher for the book that he has excessive stock of. Now, I do not see how his excessive stock has anything to do with my reimbur*****t. Not only has he lost me as a client - I tutor Mathematics and Statistics - but he will also lose many others with irrational unprofessional attitude.