The secret buyer for a plaster Picasso bust at the center of a custody battle was named this week in New York federal court documents as Leon D. Black, a billionaire financier and art collector who is co-chairman of the board of the Museum of Modern Art.

The 1931 bust of Picasso’s mistress Marie-Thérèse Walter, one of the stars of MoMA’s blockbuster show that closes Sunday, is at the center of a tug of war over ownership between the New York dealer Larry Gagosian and an agent for the Qatar royal family.