We did talk a about this a lot before I gave up but one thing I didn't really anticipate is the level of scrutiny I would be under for "non-essentials."... Specifically gym/hair/nails/lunch/clothes.
It sounds as if the conversation regarding money happened before you gave up work. In my opinion, it's your husband who perhaps had one vision of how things would be and in reality the actual is different from the theory.
In my opinion, it is important to have one's own time / money / luxuries / conveniences / treats. I was fortunate enough to have a little money set aside before I went on Maternity Leave.
One helpful thing I find is when I come back from the treat is to make it known TO my husband how important it was to me by saying things like, "Oh, it was so nice to have x done, it was nice having something done just for me, I feel happier" and that sort of thing. Being honest about the experience, my feelings and the impact it had on my overall mental health and spirit. He wasn't there so he wouldn't understand. I wake up, it's baby, then baby naps and it's stuff for the house. When I do have a sip of tea, that is cold and been microwaved twice already, I mean, where is the appreciation on that from him??!?!? Yes, he is at work earning the money however a question I ponder is how much would a salaried nanny / housekeeper / everything do-er earn keeping up our house? Would they make the doctors / dentist appointments? Would they be responsible for clothing and such?
At the same time, I will sometimes start this conversation with, "Have you done that swimming / runnning / cycling / gardening that you wanted to do?" (These are all his hobbies.) And making sure that I have facilitated him going out to do whatever it is he wanted to do. This sometimes frustrates me as on Saturday morning I would rather he be around to help with baby so I can get on and do other things however I've come to realise that relationships are about trying to balance it all out with tradeoffs over time.
Overall, start as you mean to continue. It will be challenging to change his mindset and practise on this issue now though I think it can be done.
It might be helpful identifying what are his reasonable expectations of the spending? Does he want a full cost breakdown each time you buy a sandwich from Pret? What does he actually want when he is scrutinising? It may just be better to have £650 for the house and that last £100 to spend as you see fit.