Sandra,
I really feel for you. I know how hard this is.

Here's what could happen:
1) Bull could be put to sleep now.
2) Bull could have the surgery successfully and have a new loving home for his remaining life and receive great medical care (at least until some other ailment does him in).
3) Bull could have an unsuccessful surgery and not make it through.

What would be the best thing for Bull?

Seems to me all 3 options involve Bull not suffering a lengthy great deal of pain. That's good.
So then...if you were Bull which option would you choose?

If I were Bull, I would choose option #2.
Option #2 also has the advantage of not costing you any more.

One other thing to consider:
Any of your options are going to involve you crying a lot and feeling absolutely miserable. So you really can't avoid that aspect of it. So with that in mind, try to think of what is best for Bull, not for you.

I'm just trying to help you see the situation from a more objective angle of not being inside it. Believe me, I know how heartbreaking it all is.

Sending hugs and good vibes out to you and yours.....