credit each year to every party which has entered into or which shall enter into a long-term contract pursuant to subsection (e) of
section 485h of this title so much of the amount paid by said party on or before the due date as is in excess of the share of the operation and maintenance costs of the project which the Secretary finds is properly chargeable to that party. Credit for payments heretofore made under any such contract shall be established by the Secretary as soon after
July 2, 1956 as it is feasible for him to do so. After the sum of such credits is equal to the amount which would have been for repayment by the party if a repayment contract under subsection (d) of
section 485h of this title had been entered into, which amount shall be established by the Secretary upon completion of the project concerned or as far in advance thereof as is feasible, no construction component shall be included in any charges made for the furnishing of water to the contracting party and any charges theretofore fixed by contract or otherwise shall be reduced accordingly;