Is it permissible to change the destination of donation funds allocated for food and medicine in Syria to purchasing weapons for the Free Syrian Army, after the donors' intention was for humanitarian aid?

It is not permissible to spend donation funds on anything other than what the donors intended, because an agent must adhere to the condition set by their principal. If it is possible to spend them on what they were collected for, that is obligatory. If one wishes to spend them on something else, permission must be sought from the donors. If it is impossible to seek their permission, and it is also impossible to spend the funds on what they were collected for, then it is permissible to spend them on what is more beneficial.