A group led by Symphony Technology Group managing partner William Chisholm has agreed to purchase the Boston Celtics for at least $6.1 billion, which would make the sale the richest in U.S. major pro sports history, a person with knowledge of the negotiations said Thursday. Sportico and ESPN were among those first reporting the agreement for the sale of the reigning NBA champions. Boston Basketball Partners LLC announced last summer that it intended to sell the majority of the Celtics either late last year or early this year, then would sell the balance of its shares in 2028.