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Two British museums have announced they will buy “The Finding of Moses,” a 17th-Century painting by Nicolas Poussin, for $13.1 million to keep it from being sold to the Getty Museum in Malibu. The joint purchase by the National Gallery and the National Museum of Wales is the first agreement of its kind in British museum history. Under British law, works of art more than 50 years old, worth more than $14,500 and sold to foreign buyers can remain in the country if British dealers can match the purchase price. “By acting together we have been able to achieve what neither of us could have managed alone,” said Hywel Rees, a spokesman for the Welsh museum. “We are delighted for the National Gallery and the National Museum of Wales. The British export process works effectively and we are happy to abide by it,” said Philippa Calnan, a spokeswoman for the Getty. The 1651 painting by Poussin, a Frenchman, would have been the Getty’s most important 17th-Century painting. It will be displayed alternately at the London and Cardiff museums.