Mother has dementia she looses everything and then says someone must have stolen the item whatever it is and she hides money forgets where then says someone took it she had sold her home so has some money tells me I can't tell her how to spend I don't try but she just can't remember if she has money it not my question do I try to reason with her or try to explain how it is and her getting mad.
I wouldn't bother trying to explain things like missing items. She won't likely believe you and if she does, she'll forget the next time it happens. With my cousin, I would just look and find the item. Just say it must have gotten placed there by mistake.
With my cousin, I'd put some items out of her sight, because she would likely hide it later and I would have to search, so I leaned to put the remote away, the phone, etc. If not, she would hide them and deny it.
I learned that a simple, answer that pleases and lets us move on is best. Yes, there they are, they must have been placed there by mistake, but my that blouse is really your color. I really like it. Would you like a snack or to watch tv now. I'd not dwell on it and switch the conversation when you see she's getting agitated. And if agitated, soothe her by telling her things have been corrected. All is fine. No need to worry, I took care of it. That seems to work better than trying to convince her that she is mistaken.
She hides things to keep them safe -- she is worried about theft. Then she can't remember that she hid them and that convinces her that someone really is stealing from her. It is a vicious cycle! It does not last forever, but while it does it can be frustrating for all concerned.
Don't argue with her but offer more comforting explanations. After a while you'll learn her favorite hiding places and that will help. "Oh Mom! I'm so sorry that your lovely sunglasses have gone missing! I can't imagine that Dorothy took them, but maybe she did move them when she was here. I'll help you look for them."
"Do you have money? Well, Mom, you are not rich, but by being careful you'll be able to pay your own way and not wind up in the poorhouse! And you can even afford some nice treats once in a while. I'd say you are lucky!"
This is a very challenging phase. Remember that your mother can't help this behavior, and reasoning does little good. Try to stay upbeat and sympathetic without agreeing that someone is stealing from her.