Based on my experience, you also need to consider the emotional burden on your mum if you leave your dog with her. My mum adopted our cats when we left the UK. She always claimed they were still ours but they were all so attached to each other. This week we learned that one of the cats had developed aggressive bone cancer and had to be let go. My mum sent me an emotional email to tell me, but also said how she thought she'd let us down. She was in bits about the whole thing. I felt guilty for having to put her through this, just because we didn't take the cats with us.