Is it permissible to dispose of the meat of the "sacrificial offering" left for a friend who has not yet arrived—such as by giving it to someone else or cooking it for ourselves—knowing that he is unaware this portion has been specifically allocated for him?

There is no harm in giving the aforementioned meat as charity, because it was not gifted to the friend, but rather prepared for his hospitality upon his arrival. It can be disposed of by giving it as charity to others, and if the friend arrives, he can be honored with whatever is available. Some scholars have stated that if food is prepared for a beggar and then he leaves, it must be given to someone else.