The people who whine about NATO members "not paying enough" seem to think all the other NATO countries have got rid of their armies and are defended only by US troops permanently stationed there. That's not how it works: They all have their own defense forces. Trump was whining that those are not big enough (and he was probably even [at least mainly] right there), but that just means that if there is a war in any NATO country, it will only be able to do "too small a part" in its own defense, and the other ones (yes, given their size, mainly the USA) will have to help more than they otherwise would. But until that happens, this "missing money" doesn't actually come out of any US budget; it's just not being spent at all.
For once, not even "just another one of Trump's lies", but certainly a huge over-simplification by / for the more naïve of his followers.