What should i do if my dog is about to be put to sleep?

we recently found out my dog had a tumor in his thigh and the vet said it cost 120$ to remove it but hes not sure if it will grow back or not so then it cost 140$ to send it to some kind of labratory to examine if its a bad tumor or not. and it just kills me to know that she(my dog) doesnt know about any of this.and my dad doesnt want to pay for it because we dont have enough money. but he says that hell rather put her too sleep instead of suffering but i dont know how to deal with it. please help!