(a) It shall be the duty of any purchaser of oil or gas to pay the royalty interest at the same time it pays the lessee or producer. However, the parties may expressly waive the time and manner of payment in writing.
(b) The purchaser shall at some time not later than the twelfth day of each month furnish each royalty owner with a statement showing the correct amount of oil or gas purchased during the previous month together with the correct amount paid each in interest therefor.