Is it permissible for someone who intended to give charity to a specific entity, but was then unable to do so, to give it as charity to another entity, and would that suffice for his initial intention?

Charity (Sadaqa) is not binding merely by intention. Therefore, it is permissible for someone who intended to give a sum of money in charity to spend it on someone other than the person they initially intended, or to withhold it, as long as the intended recipient has not taken possession of it. This is because they do not own it except through possession. There is a consensus (Ijma') that if a person intended to give money in charity and began to do so, but only gave out a portion of it, they are not obligated to give the remainder in charity.