ANSWER:

The girl was born on 29 February.

Thus her birthday came once in four years only while her sister was born on a normal day and celebrated her birthday every year.

Thus the girl had a chance of depositing money only 10 times in 4 years through which she collected $2500 while her sister took $50 from the piggy bank every year making the total amount to be $2000.

Thus when the piggy bank was opened, it had just $500.

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