The head of the Philadelphia Housing Authority can relate better than most housing officials to the mortgage default problems facing many Americans. That's because Wells Fargo Bank has foreclosed on his $615,035 condominium in the upscale Naval Square development in Philadelphia. The amount in dispute is $386,685.22, according to Wells Fargo. As the PHA's executive director, Carl R. Greene is in charge of the country's fourth-largest public housing agency and is one of the city's highest-paid public officials. His salary is $306,370, and he received a $44,188 bonus last year. A spokesman for Greene said that he is involved in a dispute with his mortgage company. Greene, 53, has earned respect and praise for his efforts in reviving Philadelphia neighborhoods.