real-brisker — There is nothing in bitachon that I know of that says Hashem will give you enough money to buy whatever you want whenever you want it. In fact Bitachon davka means that you trust that Hashem knows what He is doing and will give you what is right for you — but not necessarily what you think you need, and certainly not everything you want. So to spend money without cheshbon does not seem to connect to bitachon in any way in my mind — if anything Bitachon would seem to imply that if Hashem hasn’t given me the money for a specific item, obviously I am not intended to have it.
The only thing I can see as possibly fitting into a bitachon issue is how far ahead you are planning, and your attitude with regard to that planning. Meaning I can perhaps hear that planning too long term in the future may reflect a lack of bitachon in the fact that Hashem can turn things about at any moment of time, and we should not think that our planning is what ensures a financially secure future. So perhaps someone on a very high level of bitachon who therefore requires less hishtadlus may be exempt from the hishtadlus of saving long term (consult with your Rav if you think that you are on that madreiga), but that is a completely different concept then just spending money and assuming Hashem will give you it because you think you “need” it.