The National Gallery of Canada wants to buy a Jacques-Louis David painting with the money it will get by selling a Marc Chagall piece.

Questions were raised when the gallery announced it was looking to sell The Eiffel Tower by Chagall at an auction next month.

The gallery would only say it would use the expected $6 to $9 million the Chagall piece will likely fetch next month to buy an important work of "national heritage" that could leave the country.

On Monday, the gallery revealed what that painting is: Saint Jerome Hears the Trumpet of the Last Judgment, painted in 1779 by French artist Jacques-Louis David.

Saint Jerome Hears the Trumpet of the Last Judgment by Jacques-Louis David. (Musée de la civilisation du Québec)

The gallery said in a news release it has the most comprehensive collection of French art in Canada, but a "glaring exception" is an important work by David.

"To adequately represent this insurmountable exponent of Neo-Classicism would be a boon to any collection of European art," it said.

No sure thing

The painting has been in Quebec City since 1917, but in 2016 the National Gallery was approached with an offer to buy it.

By last fall, the gallery said they had heard a foreign museum was very interested in buying the David piece and had the money to do it, which added a sense of urgency and led to the decision to sell the Chagall.

On May 15, Christie’s will offer Marc Chagall’s La Tour Eiffel as a highlight of its evening sale of Impressionist and modern art in New York City. The canvas, from the National Gallery of Canada, is being sold to benefit the Acquisitions Fund. (National Gallery of Canada/Christie's)

There's one potential catch — Quebec City's Musée de la civilisation has first right of refusal to buy the David piece until mid-June 2018.

The National Gallery said they don't believe that museum is interested in buying it, but if they did, the money from the Chagall sale would be used to "improve the national collection and, especially, to strengthen Canada's ability to protect its patrimony from exportation."