The prepayment is probably like the 2017 agreement, which was sold at a floating price ie they earn the prevailing market price at the gas delivery date.
Macquarie pays CTP based on todays price less the interest cost and a haircut (ie prepayment price) to manage their credit exposure. CTP delivers the 3.5PJ over 2022 & 2023 and earns the difference between the prepayment price and the prevailing market price at delivery. Generally the prepayment price is set sufficiently below the market price to ensure CTP doesn't owe Macquarie at delivery. In the case this does happen, CTP can deliver additional gas to true up the difference.
