I agree. and with @LookSoPerfect s post
You are the mum. I don't blame you at all. Parenting is difficult and this has slowly developed and is now a problem. Its hard dealing with screaming and I really do sympathise.
But this is what is now happening.
You ask her not to keep her money under the pillow
She has a tantrum and so you allow it.
She wants things when money is tight and should use her money - yet you are agreeing to purchase it, what you can't afford
Because she has a tantrum.
A tantrum, as I think you've described this, is different to genuinely being upset, or scared..so you need to make sure its not that by talking when things are calm. But this kind of tantrum sounds like its a statement of will. "I will do what I want and if you don't let me I will scream and scream until I'm sick."
As Garlic bread said. If she has a tantrum about this after you've talked to her.
Let her,. and again and again
And stay firm. It won't be easy, but wavering doesnt work. You've still got to show her that you love her but that this behaviour won't get her anywhere now that she's getting older and that it is not a fair way to treat you. Ask her if she'd like you to scream and shout every time she asked you to do something - like buying something in a shop. Ask her (not in an angry way, or with "tone" but in a very fact of the matter, curious sort of way, what she would do if you did that. Try and make it a bit ridiculous or funny without mocking her. Even find some funny tik toks of adults tantrumming or toddlers. Something you can both laugh at or talk about. Without saying "That's you that is. "
Or if you put the food money in the fridge where cousins could steal it...I'm sure you can think of better examples but I think asking her to come up with solutions helps make her think about whether things will work or not..and gets you into conversations rather than show downs.
Forgive me if I am wrong, but it does sound like you are a little bit scared of situations where she will kick off and go apeshit. And you tread carefully to avoid it. I think you need more expert advice if you feel this might be the case.
In my experience, I'm not saying its the only way, only what I've observed over the years, but the more success she has at this method, the more she will employ it for other things and it will start turning into a real problem.
She may use this method (creating a standoff by outright angry refusal) in other ways of doing it as she gets older... not speaking, refusing meals etc..refusing to dress for certain occasions etc. so I think its good that this has come to a head for you now, so that you can nip it all in the bud by showing her that talking about when things are calm and accepting that its OK to discuss and consider the various practical methods of dealing with the problem.
In practical terms.. there are loads of children's card/app/digital accounts out there. I've heard of Henry. but most banks have them. She gets her own grown up card. A small lock box with combination lock. to keep it in, under the bed with any loose change,. And as @GlosGirl82 said, print out the statements for her.
Get her a little purse and ask how much she thinks a Halloween costume will cost and put the amount in her purse before you shop and say that means you can choose what to get. Tell her she needs to bring it just in case you don't have enough money on your card, but she can earn it back by doing things to help at home. And you will donate £5 or whatever to her savings fund if she does this. Would it be useful to set up a pocket money system, with a notebook?
Hopefully you can get over this blip. I'm sure you can think of better examples. I think you asked for help and you clearly want to solve the problem, so you are doing your best and that is a great starting point