A homeless man found a backpack containing a large sum of money and passports in a Dorchester mall on Saturday and turned it in to police officers, according to a statement from the Boston Police Department. The backpack contained $2,400 in cash, $39,500 in traveler’s checks, passports, and various personal papers. The man flagged down police officers patrolling South Bay Center, the statement said. Officers took custody of the backpack and returned it to a man who reported losing it inside the mall’s Best Buy store. Police did not identify the homeless man.