When is a Muslim permitted to forsake his Muslim brother for more than three days?

It is forbidden to forsake a Muslim for more than three nights, except for a legitimate Shar'i reason, such as forsaking the people of innovation who openly proclaim their innovations, and forsaking those who openly commit sins, as the Prophet, peace and blessings be upon him, did with those who lagged behind from the Battle of Tabuk. Similarly, forsaking a disobedient wife should only be in the conjugal bed. Forsaking is a remedy resorted to if its benefit outweighs the benefit of maintaining connection with the forsaken person. It is permissible for a Muslim to return a greeting, even if in a low voice, if he sees benefit in doing so.