But in your case you're referring to a sick animal. The question is as to euthanizing a healthy animal you can no longer care for due to financial or other restraints.
If it's a fairly young and healthy horse, there shouldn't be too much problem selling it (at a loss, probably) or at least finding a home to adopt it.
People do need to plan ahead a little, and start exploring other options
before you have absolutely no resources to care for it anymore
But a 20 yo horse, with some issues that prevent riding it much or at all - then it's hard just to keep feeding it when times are lean. The horse might live another 8 years, but it's not much use except as a pet or a companion to another horse.
Now myself, I would never want to send a horse to slaughter if I had another option. I would just put it down if I couldn't keep it or find another home for it. Montana is still pretty reasonable in that no one will question your decision to euthanize unless you abuse it otherwise.
Before we put the first mare down, we adopted a 17yo arab mare that had never been ridden. The owner had died and the survivors weren't taking care of her feet. Almost six months later, we have her feet almost healed and she is carrying moderate weight in packs. Hopefully, Ladypine will be riding her by next summer.
We don't make a practice of rescuing horses, but the timing was right.