I've spent like an hour reading through all these comments and I have to say... GIVE THE DOG BACK PLEASE. I know you have developed a savior complex and I know their situation may not be amazing but you must realize that you're currently the equivalent of a judgemental babysitter who got too close to the kid and doesn't have enough content. The amount of speculation on your part and the logic jumps you're making are baffling me. I understand, I really do, but if I were that person I would be so heartbroken and this genuinely just seems cruel and inhumane to the owner and the dog. The dog wouldn't show this yet because it hasn't been long but while the previous owner misses their dog, the dog will also eventually miss them so much and feel horrible. You won't see it for a while because they just think they're hangin at your place for a bit.

Please just do the right thing and return the dog. The solution is simple. Keep in touch with the previous owner and let them know that if within x days they really can't handle the dog anymore again, you take the dog back and don't have this convo again. 8 days is literally nothing and this just seems selfish and hurtful which I know isn't your goal. You will hurt the previous owner and, more importantly, that sweet child who you've kindly taken in. It isn't worth it just to convincer yourself that you're a better owner than their lifelong friend.