Doesn't sound like the shop will give in on this one and will probably just dig their heels in.
Why wont the shop return it to Asus? What reason did they give? It makes no sense for them not to do this as they are losing nothing...makes you wonder whether this mobo was bought legit. Did you speak to the manager/owner or just the saturday day time helper?
If you want to play hardball with the shop you could always ring your local trading standards and have a chat to them.
I would suggest contacting Asus again and insist the mobo was faulty and you want a replacement. Just tell them the shop is refusing to do anything about it at all. Asus must have a returns policy and as they are the manufacturer I think they are ultimately responsible to replace faulty goods.
I have had to do this with a Microsoft mouse before when a local shop to me went out of business just after I bought it from them. Microsoft sent me a replacement mouse within about a week.
I would think Asus have a similar returns procedure.