The above question wants to analyze your writing ability and imagination. For that reason, I can't write this story for you, but I'll show you how to write it.
First, you should think about why this character got the money in the mail and how the character figured it out.
After that, you can write your story as follows:
Start with the character receiving the money in the mail.
Show the character confused about this event and communicating with some people commenting on what happened.
Show the character by contacting the post office to find out who sent the money.
Show the character by initiating an investigation into this money and its sender.
End with the character figuring out what that money means and who sent it.
You can find more information on how to write a short story at the link:
