All expenses that may be incurred on account of the government of Puerto Rico for salaries of officials and the conduct of their offices and departments, and all expenses and obligations contracted for the internal improvement or development of the island, not, however, including defenses, barracks, harbors, lighthouses, buoys, and other works undertaken by the United States, shall, except as otherwise specifically provided by the Congress, be paid by the treasurer of Puerto Rico out of the revenue in his custody.