All fees, fines, costs, and forfeitures as would be deposited to the credit of the United States if collected and paid into a district court of the United States shall become revenues of the United States when collected and paid into the United States District Court for the District of Puerto Rico. The sum of $500 a year from such fees, fines, costs, and forfeitures shall be retained by the clerk and expended for law library purposes under the direction of the judge.