the nickel WAS yours, it was given to you, at least would have been yours as soon as you picked it up. it was no one elses in any case. if you saw it it was designated for you.
which is why many Talmedei Chochomim advise picking up a coin one finds (some argue on which minimum amount, some say any money) if no one is able to see you or if they are unable to determine what you are picking up. its a gift from Hashem