If the clause is that high then I feel the player will definitely remain at Chelsea until they decide to reduce it. Chelsea knows what they are doing and is like they want to try their luck and see if any club can come for him but deep down they know Jackson is not worth that amount.
Manchester United need to be serious on the next player they are going for and if by chance they want to get the signature of Jackson then they should be ready for what comes next. The Blues has already made it clear that the player can't be a starter like he used because the new players that are in are very serious and they mean business so no jokes anymore, the highest that can happen is a long term loan for Jackson.
Additionally, Chelsea can equally loan him out just as you have rightfully said, it might not be to Manchester united but if truly they wish to Carry on the season without him, then he might goes to Strasbourg, Chelsea's sister club or any other team interested in him.