He seemed really surprised though that I went back to him and told him about his mistake and then gave him the extra 2€ back. For some reason most people don't seem to do this.
Personally, I like money as much as the next girl, but a few years ago, I was coming back from work by train, carrying the day's proffits from the shop I was keeping open for my dad. Point being, I had all this money in a small plastic bag with no identifiers on any part of it and it must have slipped out while I was taking out a book to read. There was over half my pay for that month in the bag, so losing it was a huuuuuge thing.
When I went in to report losing it, I didn't exactly expect to get it back, unmarked plastic bag, lots and lots of money... most people would have just kept it. Luckily for me, the guy who found it was more honest than that. The money was found by what's the word in english, the guy who checks your tickets. He brought it in with lost objects and all of the money, to the last cent was still in it.
If he hadn't, I would have had serious money troubles that month. I decided after that to always try and be honest about things like those and pay back the good karma I got that day.
So yeah, I'm probably a bit silly, especially with small amounts, but it's what I do.
In other words, far too few people offer their seats to pregnant women or the elderly on the tram. Not that they always accept when you do offer though... But it's always nice to give them a chance to accept.