Henderson has had many loan options already as a United player. He's obviously progressed as they hoped and he hoped for.
His new wage will reflect his standing as the heir apparent to DDG, whether that's in 1 or 2 years time. The future at United for Henderson is very soon, but continues to become reality year by year.
Henderson has to play week in and week out and to perform. If he's battling DDG as the number 1 this coming season and they continue to rotate, then fine. But he has to play consistently in order to get momentum for Euro 2021. But if Henderson comes back to United, then United currently have 4 senior GKs in DDG, Romero, Henderson, and Grant. Again, Grant and Romero's contracts expire in June 2021. Only DDG and Henderson are contracted after June 2021, and that's not considering a new contract for Henderson.
Most clubs insert a right of first refusal instead of buy back clause, those are two different things or can mean two different things.