I would not use waiting for lists for this. That's reserved for things that are on somebody (or something) else's plate. For me, waiting for's are not things I do, but trigger of things others do.
If it's something you're definitely going to buy and your system lets you create actions with a future start date I would create an action there if it's clear you'll have the money by a certain date. If your system doesn't let you do that, then a quick note on your calendar is ok also.
If you don't know when you'll have the money or you're not certain if you'll buy it when you get the money, then I would put it in a someday maybe list which you'll see in your next weekly review.