Yeah, if that’s what happened it is unethical because, having signed the document; Dave is effectively in limbo - he is committed but the club are not which seems as if it ought to be illegal. Dave’s obligation probably could be voided in court, if the offer / contract wasn’t withdrawn, but unpleasant for him regardless. If this sort of malarkey was unchecked, clubs could routinely offer players favourable terms to get them signed up (dissuade them from joining other clubs) and then withdraw at the last minute, offering a much worse deal.
Still, if United withdrew then Dave is not bound and can do what he likes. I suppose the extent to which this is unethical or immoral depends on how long we kept him on the hook with a one-way commitment. It also depends on the actual facts of the case, which have been subject to contradictory press reports and general speculation.
After reflecting on this a bit, there is a lack of honour and integrity from the club in not standing by their offer. But I don’t have that much sympathy for Dave, really, because it sounds like he played hardball when a little more humility was appropriate. And I don’t see that he is in a worse position now than if no offer had been made at all.
He’s been highly paid and and has done his best. His time as no1 at United is over; all good things come to an end. All said and done, it’s a bit of a mess but it’s not a big deal.