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.
Let's say that if Onana does not have much of a market, it is primarily the fault of his performances on the pitch in the last two years, and unfortunately also the fault of the management of Manchester united who overpaid this player after only one good year played with Inter. For goodness sake as a player he is good, especially for setting up play with his feet, but as a goalkeeper he often leaves a lot to be desired. In summary 57 million spent badly for Manchester united