similar thing happened to me last year.
as much as i wanted to keep the dosh, i realised that these things will always get discovered sooner or later. so rather than have to explain why i had kept the money, i went to the accounts dept myself to inform them of their error. needless to say the girl in question was grateful for my honesty (saving her blushes when said error discovered) and since then i have not had one grumble or query about my expenses since.
swings and roundabouts.