Dischem refuses to credit my mom

My parents Doctor sent Dischem a script for my father. My mother had no idea what it was and arranged delivery. When the lady delivered there was an Oxymeter for R630 which was faulty and which my parents in their 80s have no money for. My mom asked the lady to please return and after a telephone conversation the lady said it will be replaced. My mom said no she doesn’t want it but the lady rudely insisted it will be exchanged. My mom said she will then take it back when she has time and get her money back. I then immediately sent them one I had bought per courier. My mom then went into Dischem when she was able to and they refused point blank to credit it for her. It was never used nor is it needed but they desperately need the R630 my mon had to scrounge around to find. Their car has just broken down and my dad now needs a back brace for over R2000 which they do not have, but an Oxymeter is lying there worth R630 thanks to Dischem!!!! I did send a complaint 2 weeks ago straight to Dischem but not surprisingly haven’t heard a word back. So disappointed in Dischem to do this to 2 old people who have no money!!!!