American Express has contributed $150,000 for a joint production by the Washington Opera and the Orchestre de Paris, it was announced at a press conference in the Kennedy Center yesterday.

The largest corporate grant in the history of the Washington Opera, the $150,000 will pay about half the cost of sets, costumes, props and transportation for Mozart's "Cosi fan tutte," which will be conducted here by Daniel Barenboim next November in his American operatic debut.

The same production, designed and directed by Jean-Pierre Ponelle, will have five performances in June, with Barenboim conducting the Orchestre de Paris, at the The'a tre des Champs Elyse'es in Paris. The cast was not announced at yesterday's press conference, but Martin Feinstein, general director of the Washington Opera, said plans call for substantially the same international cast in Paris and Washington.

Among those who joined Feinstein at the light-hearted press conference were Meredith M. Fernstrom, senior vice president of American Express for public responsibility; David Lloyd Kreeger, president of the Washington Opera, and Peter Diamond, artistic counselor to the Orchestre de Paris.

The grant "almost didn't happen," Diamond joked, because Barenboim let payments on his American Express card become overdue. "I am absolutely sure that Daniel Barenboim will pay his American Express bills in the future and allow this relationship to continue," he said.

Kreeger recalled a recent bill he paid in a Paris restaurant "that far exceeded the amount of the grant," and Hugh Southern of the National Endowment for the Arts remarked that Mozart, with his uncertain income, "would not have been qualified for American Express."

"Cosi" will be the first of three Mozart operas to be conducted by Barenboim in Paris and Washington. The others will be "The Marriage of Figaro" in 1984 and "Don Giovanni" in 1985. The grant is for this "Cosi" production only, but Kreeger said American Express will be given the first option for support of later productions.

"We hope," said Feinstein, "it is the beginning of a long collaboration which will go on for many, many years."

The American Express Foundation, established in 1980, has underwritten many traveling art shows, including the El Greco exhibit that was at the National Gallery last year. But this is the first time it has subsidized an opera production. Fernstrom said the foundation prefers to support artistic projects that "cross international boundaries." A joint French-American production of an opera in Italian by an Austrian composer, under the direction of an Israeli conductor, seems to meet that requirement.

After the performances, the production becomes the property of the Washington Opera. Feinstein said several other opera companies, in the United States and Europe, have already expressed interest in using it.