When will my DH learn that you should NEVER promise a child anything, ever? Especially a puppy.

DH is in the army reserves. He had a lab puppy years ago, for several months; SD loved the puppy. Then he deployed so he gave the puppy to his cousin several states away. He comes back and promises SD he will get her another puppy someday. He also promises SD he will never deploy again. What happens a year and a half later? He deploys again.
Two months ago SD broke down crying to DH that she doesnt have a puppy, she's always wanted one, she's soooooo lonely, he promised her one years ago, etc. DH goes into Disney dad mode and promises to start looking for a puppy for Christmas.

This disturbed me greatly.
1. We have a 5 month old DD. SD (age refuses to get anywhere near the baby because, "she might spit up or drool on me". I have no idea how SD will cope when she has to clean up dog pee, dog drool, etc.
2. I got laid off in October. I am going back to school mid January. My DH works, and sometimes travels for his job. He also travels for the reserves on his drill weekend. Right now I watch the baby constantly and drop off the baby the next state over with my parents when we both have drill the same weekend (I'm reserves as well). What will happen to said puppy when we both have drill? DH said, "we can work it out" and "the neighbors can feed him". No. A dog is a family member. You plan for a dog. You do not pawn your dog off onto the neighbors once a month.
3. Puppies are time. DH already complains that he doesn't have enough time in the day. Okay. During the school year SD is here every other weekend and one weeknight. When it's summer, it goes to every other week. It would make more SENSE to get a puppy in the summer when SD is here more often so she can be primarily taking care of it. But gasp...common sense is lost on these people. I told SD she will probably be upset only seeing her puppy for a weekend and then having to go back to her moms and not see it. SD said no, she trusts we can take care of it while she is gone and both her parents have FaceTime on their phones.
4. Dr Evil (hex) is PAS queen. 6 months before she and DH divorced (SD was 1), she told my DH he needed to get rid of his dog because she was "allergic". DH regrets not getting rid of hex right then and there instead. I am afraid that eventually SD will get tired of the work that a dog entails and because "allergic". SD hates chores (she has a maid at her moms) and will makes medical excuses to get out of sweeping "my palms are too itchy". I know her mom will make PAS comments about the dog. SD will then refuse to pick up after the dog, or walk it because she will be "allergic like my mom". I don't know how realistic that scenario is, but I have this gut feeling that I will be the one mostly caring for the dog.

So I think if we are getting a dog, it should wait until summer. SD (might possibly) be more mature and will be here to take care of it. DH agreed with this last night. I told him he needs to go TELL SD she is not getting a dog for Christmas because that is what she believes, and what he has lead her to believe. He says he will "do it later" which is code for "I'm not having a conversation that will end badly with my daughter". But he needs to tell her or else she will wake up Christmas morning and be distraught that she has no puppy, and DH has lied.