Questions about showings/offers

[John H.]

Whats the proper etiquette for this situation.  Western Mass, so not a busy market at all.  Selling a house, an offer came in that expires today at 5pm.  Just got an a email, another agents wants to show tomorrow.  

I want to sound as professional and courteous as I can, how do I reply to the agent looking to show tomorrow.  The offer is under asking but If I counter today, and they accept, I don't want to upset a potential buyer.  Could be a hard house to sell due to location.

Before I reply back, i figured i'd ask here to get some more ideas.

Thanks you!