Well, at European stadiums after a while a steward will come up to you with a request to give the ball. It is generally considered normal there to give the balls, often people who catch the ball throw it on the field themselves. We also have a steward to come up to you, so if he is not lazy and if you are not sitting in a "closed" sector, the ball will most likely have to be given.
Now this is your ball, take it home, sell it on eBay, save it as a rarity. Do what you want.
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