Help my cat Diablo to stop suffering

I have a 5 years old cat named Diablo, we named him that way because he was almost feral when we adopted him. But after a few months, he became really cute and cuddly. He's been with us since he was abandoned at birth. I always had enough money to pay for anything that he needed, but recently he got really sick and I got huge bills from the vet (~500€).

The vet didn't find anything with all the exams, he only knows what the issue is (fast decaying blood cells) and where it doesn't come from (no infection, no "cat AIDS", a few other disease discarded, etc..). I can't pay for more exams to find the issue, since it could go up to thousands. Diablo is suffering right now and the vet advices us to put him down to end his pain. Since we haven't paid the first ~500€ bill entirely yet (we agreed with the vet on a delayed payment in 4 times, from december to march) the vet want the payment first to put him down.

The issue is that I don't have the 140€ the vet asks right now to put him down. Mostly because it's winter and my other bills are already higher in that time of the year. I've looked at organizations that help pets for free/cheaper but they are overwhelmed with too many abandoned pets and they told me they can't help us.