Impossible is probably the wrong word. Highly improbably is more accurate.
The European away tickets are allocated fairly by the club, i.e. they go to the people with the most credits and you earn your credits by having attended previous European away games. Prior to this season, you had to collect your ticket in the city of the away game, i.e. fans would have previously had to have collected their ticket in Lyon for example.
This year has changed and tickets are now being sent either digitally, or to peoples home addresses, which works slightly in your favour as spares are becoming more prevalent. However, you are going to need to find someone who has applied AND got a ticket AND isn’t travelling AND is willing to let you, a complete stranger have their ticket. IMO, and this is just my opinion, that person is highly unlikely to exist and then you have to add in the fact that any person who fits the above criteria is more than likely to have a mate or a mate of a mate who they are more likely to psd their ticket on to over yourself.
If I had a ticket, for example and wasn’t travelling, I am more likely to speak to people I know who will be travelling and might not have a ticket before I sell it to a stranger on a United forum.
No harm in asking though. Your best bet might be one of the ‘legal’ ticket touting sites but the ticket is likely to cost in excess of £300