A Michigan gallery that has had a long and lucrative relationship with the pop artist Peter Max sued his studio on Monday, alleging breach of contract and misconduct in the sale of “several thousand works of art by Max for several million dollars.”

Park West Gallery, with sales of roughly $500 million annually, said it had an Oct. 31 contract to buy a trove of Mr. Max’s “personal selections” from ALP, the company that runs his studio and business. On April 5, the lawsuit alleges, ALP tried to rescind the sale of these works, a mix of paintings and collages that the artist had put aside because he was particularly proud of them.

“The studio was paid in full and shipped the artwork, but now is attempting to back out of a valid and binding contract,” Park West’s lawyer, Luke Nikas, said in an emailed statement.

A lawyer for ALP, Jeffrey M. Eilender, called the lawsuit meritless and said it was an attempt to pre-empt a lawsuit that ALP planned to file claiming that Park West stole artwork Mr. Max “had specifically reserved for himself and his children.”