The store owner was out of line in involving you. If the store owner is Jewish, suggest to him that he take the neighbor to a Din Torah, if he has no other recourse, but that it was Loshon Hara for him to write such a letter to you.
Now for the dan l’kaf zchus part – maybe the letter you got was bogus. How would someone happen to hit on YOUR name in a phonebook of a gazillion names, the one person who lives next door to this person he claims owes him money. But even if not, it is not right for anyone to send out such a card to you or anyone else.