.....ok.....
You give the customer YOUR money because you are going to get it back anyway (when you return the item across the street), what you are doing is correcting a mistake on the part of one of your employees, this way you have a customer that leaves happy and knows that you will stand behind your store/work...why do you care about this...well that means YOU HAVE A RETURN CUSTOMER
Now if I was running a best buy store and I didnt have the part what would I do... I would tell the cutomer: "we dont have the part but we will have a employee go get it and it will take but a second, go get lunch and your car will be done"
I would then go across the street to the competitor store and get the part, ring that part up (as extra labor or whatever to not make my inventory off).
Now why on earth would I as a manager spend the extra 4 minutes of my time when I could spend that time w/ my thumb up my azz or better yet micro-managing, well because the customer knows that you could have told him you "don't have the part we cant take your order" (but if you are that simple you should be working in fast food, though that is not all that much higher up the ladder from what I see at most BBs), by letting the customer know that you are going out of your way (if only very slightly) to help them, they will come back to you for future installs / purchase (or at least you will have a whole heck of a better chance of earning there business compared to your competitors)
short answer is..drum roll please....IT WILL MAKE YOU MONEY!
*Oh and to address your "where would you draw the line" comment, the only reason you are refunding it is because the item was purchased in conjunction w/ a audio install, I would not return a tv or what have you from another store.
*On the "if you had a business" note, I am not going to get into my personal affairs on the web w/ someone I don't know; none the less I'm not exactly starving here!