How did you find out it is indeed one of the works sold that night? did the receiver tell you the work? Anyway, I would be fine with saying "I'm sorry, this work was a final sale, part of a ____ night." (local artists gathering whatever night. )
But how about offering them the amount in a gift certificate toward another piece? That's not quite the same thing...you would be offering to work with him/her. Nothing wrong with exchanging the same value, is there? I wouldn't be at all worried about ruining buyer's rep for purchasing an inexpensive item. The recipient had no problem coming to you for an exchange! So all's fair in love and art!