Leftover Funds Must Be Returned or Credited

Q: Our association levied a special assessment after Hurricane Irma for cleaning up our property and repairing some minor damage to the buildings. The damages were not bad enough to reach our insurance deductible. The board ended up spending quite a bit less than they assessed for, and decided to put the left over money in our reserves. Is this legal? I thought the board had to return the money to us? (J.S., via e-mail)