U.S. banks are resisting efforts by state attorneys general to force them to cut the amounts owed by some borrowers facing foreclosure. Yet mortgage companies already have reduced home-loan balances for more than 100,000 borrowers. How much larger the number will grow is likely to be at the center of negotiations this week aimed at reaching a settlement to the nationwide investigation of mortgage-servicing practices. Officials from Bank of America Corp., J.P. Morgan Chase & Co., Wells Fargo & Co., Citigroup Inc. and Ally Financial Inc.’s GMAC unit have been summoned to Washington for a Wednesday meeting with state attorneys general and at least three U.S. agencies, according to people familiar with the situation.