"Money can't buy happiness" is so misunderstood and often misused.
The poor use it as a kind of "well I didn't want it anyway" sulk - y'know like "bitter (not bitter)"
The rich use it to diminish the real value of it, to take the heat of themselves, insinuating that they are not happy (if they are not, having money is unlikely the source of it).
The reality is that money can't buy happiness but it can buy all the things you need to be happy, and especially avoid the stress and worry about money that often makes people unhappy.