This phase I trial is studying the side effects and best dose of RO4929097 when given together with letrozole in treating post-menopausal women with stage II or stage III breast cancer. RO4929097 may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Estrogen can cause the growth of breast cancer cells. Hormone therapy using letrozole may fight breast cancer by blocking the use of estrogen by the tumor cells. Giving RO4929097 together with letrozole may be an effective treatment for breast cancer.