Is it permissible for the person authorized to receive and deliver funds in a monthly society to use some of the amounts paid to him early due to travel circumstances and keep them with him until he delivers them to the rightful recipient at the end of the month, knowing that he pays them in full without delay?

The questioner is an agent for the person who paid him this money to deliver it to its rightful recipient. An agent must adhere to the limits of the agency and not deviate from the authorization of his principal. If the owner of the money did not authorize the questioner to use the money for himself, then he is not permitted to do so, as it is not permissible to dispose of another's property without permission.