She found the piece was signed by Fleming, who died in 2018.
Manthey bought the sculpture for $39.99, but she said she was sure it was worth far more.
She decided to donate the piece to the Birmingham Zoo's annual gala auction.
"This unique sculpture with a rich green patina captures the magnolia flower in full bloom," the auction description reads. "Fleming primarily worked with bronze, a material that allowed him to capture the fine details of his subjects while also ensuring the longevity of his sculptures."
Bidding for the sculpture starts at $2,000. Bidding closes Oct. 5.