I don't think if Manchester United has plans regarding Onana next season, what if they even sold him to sign a goalkeeper whom they will spend less but give quality keeping because the amount they spent on him is very high, so actually as Manchester United is badly in need of money they shouldn't have or keep someone that's receiving high but given less to the club and truly Onana is given less to Manchester United, perhaps Amorim has not thought about his situation yet which means there will be next season for Onana and Manchester goal post will remain unstable as always.
What I know is that Ruben Amorim will listen to every offer that comes in, and that means if there is an offer for Onana, I think Amorim might sell the Cameroon goalie. But it also does not preclude the possibility that Amorim will buy a new goalkeeper without having to sell Onana first, because as far as I know that Sir Jim Ratcliffe is aiming to sell his other company (OG Nice) and he will be used to fund Man United. If that really happens, then Amorim will most likely get quite a lot of financial support.
Definitely if there is an offer for him he will consider it very well but the consideration wouldn't be if Manchester United will be losing money from him because high offers come from quality and when there is no quality higher offer will be reduced to the length the players is and is obvious there wouldn't be high offer for him because already clubs who are considering of replacement of goalkeepers are not much because keepers are not always the problem but for Onana is a big one, so actually if there is a big offer Manchester United coach wouldn't think twice.