A Russian businessman with ties to the Kremlin paid hundreds of thousands of dollars to help finance Donald Trump Jr.'s wedding after-party in the Bahamas this year, according to two people with knowledge of the matter. The payments represented an extraordinary flouting of ethical norms for an American president's family.

The concern, ethics experts say, is that a foreign actor could ply a president's family member with gifts and eventually expect something in return. Mr. Trump, the president's eldest son, accepted the gifts from Umar Kremlev, the president of the International Boxing Association, according to the people with knowledge of the matter. The payments, reported earlier by Pro Publica, included the cost of renting a private island and a fireworks display.

After Pro Publica reported on Mr. Kremlev's payments, Mr. Trump and his new wife, Bettina Trump, posted a statement to Instagram acknowledging the gifts. They said that the trip to the island pertained to “an already planned weekend” with friends, not their wedding ceremony, though some ethics experts said that distinction was irrelevant. They also portrayed their relationship with Mr. Kremlev, who did not attend the wedding ceremony, as nothing more than a friendship.

President Trump did not attend his son's wedding or the after-party. Asked on Monday whether the president knew that Mr. Kremlev helped pay for the after-party, the White House provided The New York Times a statement from the president, who said: “I have no idea who Umar is, never heard of him, and he didn't pay for Don and Bettina's wedding, which took place at a totally different location, and on a different day.”