Help Gambit Overcome His Health Struggles

Hello, my name is Anna and this is my dog Gambit. For a long time he has been struggling with an unknown skin allergy and a mass growing in his leg that is starting to effect his walking. He has been checked and thankfully his growth isn't cancerous. He has had these removed before but this one is the worst one. It's going to cost $800 to get it removed. Gambit is my special and boy it pains me to see him struggle. I try really hard to raise money for him but not that long ago we just had to put his sister down which was an expense I wasn't expecting. So Gambit is all I have and I want him to live a good life while I still have him. He is 8 years old and still very young. I've tried to figure out what it is that is causing his skin allergies. I've tried changing his food, cleaning products, medicated shampoos etc but the only solution I have is to get him a diagnostic test done, which is $500 estimated by the vet. So in total he needs $1,300 to finally live as he deserves. Any help is very much appreciated and even a great start to his healing.