First, sorry to hear you're in a tough spot. I moved your post here because we've had many Tripawds go through similar situations so you're in good company and you'll find lots more feedback. Also if you scroll through this forum's topics you'll see those stories so you can read more details about how others dealt with it.

OK first off, don't panic and don't rush to any decisions today. Now, what type of vet has diagnosed this situation? Is the vet a certified orthopedic surgeon? If not, you need to get to one. Now, if the vet IS an ortho surgeon, then please see a certified canine rehabilitation veterinarian. Another opinion can give you the perspective of a practitioner who doesn't jump to surgery and tries other healing modalities first. Here's a good article to read about trying other methods before surgery:

Treating Soft Tissue Injuries in Pets

My other feedback is this: if your vet jumps to a euthanasia recommendation because of this, you need a new vet. There are many options for treating this situation and tons of good vets who can help you through it. Also, you have every right in the world to take your pup somewhere else. There is no need to stick with a vet who pressures you into decisions.

Tell us more about the situation around the diagnosis so we can better assist. 

Last thing, IF surgery is necessary (and many times it isn't, we've seen many Tripawds get misdiagnosed and heal up with the help of therapy), this article will give you some hope that recovery can happen:

Remaining Limb Surgery Recovery for Tripawds

I hope this helps!