Sotheby's was granted the right by the city of Denver to sell four rare-to-market paintings by American abstract expressionist Clyfford Still (1904-1980). Rival auction house Christie's says it could offer the city a better deal.

Both Sotheby's and Christie's submitted proposals to handle the artworks which are being sold to benefit a new Clyfford Still Museum in Denver, scheduled to open in November. The four paintings could net the museum $25 million.

A collection of 825 paintings from the artist's estate, from which the four were culled, are to be housed in the Denver museum. The artist stipulated in his will that his artworks be given to a city that would build a specific museum for his work, keeping the collection intact and restricting any sales or loans. His widow, Patricia, chose Denver in 2004 and bequethed her own collection as well.

About 94 percent of Still's works are part of his estate, which has been largely out of public view since his death in 1980.

Sotheby's agreement includes a guarantee of a minimum net to the Clyfford Still Museum of $25 million. If another museum is the buyer, Sotheby's could get a 5 percent commission, or $7.5 million, whichever is less. If private buyers purchase the works at auction, Sotheby's stands to get a $15 million commission.

The three most recent auctions of Still's art were held at Christie's. Most recently, his "1946 (PH-182)" fetched $14 million in 2008.

According to a Christie's statement: "We are concerned that the process of awarding of the contract was arbitrary and capricious. The public will not be served by rushing into a binding sale agreement before a full and complete consideration of proposals for sale. Based upon our review of the publicly available documents, we had offered a considerably more favorable bid to the city and its citizens."

"It was a competitive process that was fair and followed the city's contracting procedures," said Jan Brennan, with the Denver office of cultural affairs and one of the nine members of the selection committee.