Poor & incorrect service

I approached the technical sales person and told him that I am in the market to purchase a WiFi router for my home. I have limited knowledge of routers so I am reliant on his advice. I told him that I have Neotel modem ****le. He advised me that I bring the unit to the store so that he can see if it's compatible with the unit he sold me. I purchased the unit that he gave me and assured me that the unit is compatible. I got home tried to connect as he explained, which did not help. I read through the manual and followed the installation set up. It gave serial codes and log in codes which i wrote on the box. The router was not compatible, when I returned it they took 30% off from the refund for the damaged box, which is there policy. But wasn't made aware that it is on the receipt. I bought the unit on the advice of a technical person. I should not be penalized for his mistake I think.