It's a very difficult one. Both because we don't really know Garnacho's full potential and also because we don't know what the funds will be spent on.
What we do know is that we have several positions that need urgent attention (GK, LWB, CF, AM and likely CM), whilst not having the funds to recruit for these positions.
I'm torn but probably end up on the side of accepting a £60m+ offer. My reasoning being that going into next season with Garnacho and maybe 1-2 other signings isn't going to put us in a Champions League position, particularly as he is unlikely to be able to flourish in the left attacking midfield spot given his skillset.
Therefore we end up with a very talented squad player and 1-2 additions to the squad, or the opportunity to go into the summer targeting 4-5 first team players.
I don't buy the argument "what's the point in selling a good player when we'll inevitably just buy a crap one with the funds" because a) if that premise is accepted then we're going to be an upper mid table team in perpetuity regardless of who we sell and b) the summer business seemed a big improvement on historical windows in my view.
I'd obviously prefer to sell Casemiro, Rashford, Eriksen, Lindelof, Mount, Malacia, Onana and Antony; but my guess is all of these put together would raise less funds than Garnacho, given their huge wages in comparison to current ability.