You absolutely HAVE to get his foot dealt with medically! You don't have the money? It is NOT an option. Any more than it would be if your foot stank, and was riddled with a tumour/infection, etc!
Find the money.
The fact that you mention he is 10 years old sounds a bit ominous! Not because there is anything wrong with a dog being 10 years old, as many have plenty of life left in them at 10!
But because... (do I hear this right?)...I get a sense that you are using his so called "age" to possibly justify disposing of your dog because it's the cheaper option. (?!)
Also the idea that there may be no point treating his foot, as your other dog had cancer in its foot, you paid the money, and it came right back...
What is that supposed to mean?
Get your dog's foot treated. Even if you have to sell your car or re-mortgage your home!!
Get the leg amputated if necessary but get him out of pain a.s.a.p.