It makes life much more enjoyable to not tolerate regret or guilt but certainly say sorry and pay reparations if needed but dont foster negative emotions...but one wonders why things like you mention happen with so many things.... You want a new battery for your car so you search for a couple of weeks then buy one and later that day your mate will say..
"why didnt you tell me I get them half price?!"
We know this will happen so just learn to live with it
Alex