Bosses at The Amy Winehouse Foundation (Awf) paid $41,500 (£25,938) for two Steinway baby grand pianos from Ronen Segev's Park Avenue Pianos store in New York last year (13), and had planned to auction them off at a gala fundraiser.
One, signed by Billy Joel, was delivered to the winning bidder without issue, but the person who fought for the Bennett piano failed to hand over the cash, prompting Awf chiefs to ask Segev to deliver the instrument to the charity's offices instead.
According to the lawsuit, Segev refused and has allegedly been holding on to the 1916 Grand Model M baby grand Steinway ever since.
Foundation bosses are seeking the delivery of the piano, or damages equal to its market value.
The Rehab hitmaker's parents established The Amy Winehouse Foundation to support troubled young people after the singer's death from alcohol poisoning in July 2011.