The Build-a-Bear was accidentally donated to the Goodwill store in Tazewell, Tennessee.
The bear reportedly contains a recording of the child's late mother's heartbeat.
"My daughter's mom passed away and her grandma made her bear with her mom's heartbeat in it," dad Tylor Kennedy told ABC affiliate WATE.
Workers at the Goodwill put a note on the door asking whoever bought the tie-dyed bear to return it for reimbursement.
So far, the bear hasn't been returned, but the family is holding out hope that it will be.