When a Swiss museum announced in September that it would be putting three paintings by Paul Cézanne, the Impressionist master, up for sale at auction, there was immediate protest by some who suggested the sale violated ethical guidelines on when institutions should sell off their works.

One of the Cézannes in particular, a still life titled “Fruits et pot de gingembre,” is a highlight of the Museum Langmatt in Baden, which houses a small collection of Impressionist works. The museum said it was financially necessary to sell the painting, and perhaps two others, to keep the foundation that owns it from insolvency. The still life is estimated to fetch $35 million to $55 million at an auction on Thursday at Christie’s in New York.

But the critics have not been persuaded.

Tobia Bezzola, the president of the Swiss branch of the International Council of Museums, called the prospect of a sale “outrageous,” “short-sighted” and a breach of his organization’s guidelines on deaccessioning museum objects.

“Bequests and donations come to museums because people think they will be safe,” said Bezzola, who argued the sale should be canceled. “All the important collections in Switzerland come from private donations and bequests, so this sends a terrible signal.”

But the museum has never wavered in its intention to sell “Fruits et pot de gingembre” (1890-93), and in recent days it has said the sale will also result in compensation for the family of a Jewish art dealer who once partly owned the work.

New research, the museum reported, had turned up evidence that the painting, which was purchased in 1933 from the Jewish dealer and his partner by the father of the museum’s founder, was most likely sold under duress. The discovery was made after Christie’s announced the sale, the museum said, and so it had recently reached out to the heirs of the dealer, Jacob Goldschmidt, to forge a restitution agreement that will compensate for the sale of the painting. The terms were not disclosed.

Mara Wantuch-Thole, a lawyer who represents the dealer’s grandsons, said they had not been aware of Goldschmidt’s ownership of the painting until they were contacted by Foundation Langmatt, the museum’s owner. She said the museum reached an agreement during “one weekend of intensive work.”

“Once there is consensus that a transaction would not have taken place if it was not for the persecution by the Nazi regime,” she said, “reaching an agreement doesn’t have to be a lengthy and difficult process.”

The director of Museum Langmatt, Markus Stegmann, said, “We are really relieved that it’s all been settled,” adding, “We thought it might not be possible in such a short time.”

The Nazis were already in power and the persecution of Jews had already begun in 1933 when Galerie M. Goldschmidt & Co. of Frankfurt, and L’Art Moderne, a gallery in Lucerne, who jointly owned the Cézanne, agreed to sell the painting. The buyers were Sidney and Jenny Brown, whose son, John Alfred Brown, would later bequeath the family’s villa and art collection to the city of Baden to set up a private foundation and the Langmatt museum. The Cézanne is one of roughly 50 works held by the museum.

Though the painting was outside Germany, at the Lucerne gallery, at the time of the sale, museum officials say that Goldschmidt was pressed for money as the Nazis made it increasingly impossible for Jews to do business. Stegmann said it was not clear whether Goldschmidt had ever received his share of the 1933 sale price: 57,750 Swiss francs, which he said was viewed as a fair market price at the time.

“One could reach a variety of conclusions in this case,” he said. “But it was clear to us that the circumstances Goldschmidt was in meant he was suffering a great deal. He really needed the money to support his family and to flee.”

Lukas Breunig-Hollinger, the president of the Foundation Langmatt, said a significant piece of evidence had surfaced after the sale was announced this year. Museum officials and Christie’s declined to detail what that evidence had been.

In a statement, Christie’s said that “in the course of our rigorous pre-sale restitution research,” the auction house and the Foundation Langmatt determined that “‘Fruits et pot de gingembre’” should be considered further to address its ownership history.”

Christie’s has a restitution department that is dedicated to researching the provenance of items that may have changed hands in the Nazi era. But earlier this year, the auction house drew broad criticism after holding a sale of jewels from an estate partly built on profits made from the purchase of Jewish businesses during the Holocaust.

Stegmann said that when the foundation sent the painting to Christie’s, it warned the auction house about Goldschmidt’s prior ownership, which the museum’s own provenance researchers had pointed to during a recent intensive review of the collection.

Wantuch-Thole, the lawyer for the dealer’s grandsons, said Goldschmidt’s Frankfurt gallery suffered greatly in the Nazi boycotts of Jewish businesses in 1933. Tax records suggest he began selling off his stock after that and rented an apartment in Paris, she said. No inventory or stock-book from his gallery has survived, so reconstructing the sales he made under duress requires painstaking cross-references with the records of other dealers he worked with, she added.

In 1934, Goldschmidt was barred from working as a dealer and he moved out of his gallery to operate from his mother’s apartment, Wantuch-Thole said. He escaped to Paris in 1936 after paying the punitive “Reich flight tax” and was interned in France from 1939 to 1941. He then fled to the United States and re-established himself as an art dealer. Two of his sisters died in the Holocaust.

Stegmann described the potential sale of the Cézannes as an “emergency measure” necessary for the museum’s survival. The foundation hopes to raise 40 million Swiss francs (about $44 million) from the Christie’s sale and set up an endowment fund that earns sufficient interest to keep the foundation afloat and the museum operating.

Christie’s, in its pre-auction announcement, described the format for the Cézanne sale on Thursday as “unique,” as it has designed a protocol to allow the museum to sell off just as much of its assets as it needs to.

The two other Cézanne paintings from the Langmatt to be included in the auction will be put on sale only if the bidding on “Fruits et pot de gingembre” does not fetch the target amount of 40 million Swiss francs. If it does, then “Quatre pommes et un couteau,” dating from about 1885, with an estimated price of $7 million to $10 million, and “La mer à L’Estaque,” dating from 1878-79, with an estimated price of $3 million to $5 million, will be withdrawn.

Stegmann said he is confident the Foundation Langmatt will still reach its target, even with the agreement to compensate the Goldschmidt heirs.

“We have to decide on the spot in the auction room whether to sell the next painting or not,” Stegmann said. “We will have to calculate it according to the latest dollar exchange rate very quickly.”