rob, your comparison is absurd. You CAN donate something you don’t have to fulfill the mitzvah of tzedaka or hachnosos kallah. i.e. You can give a poor couple a microwave even if you don’t have one – and you still fulfilled the mitzvah.
Some people, perhaps not yourself, DO like to make others happy even with things they may themselves not have or even need.
Additionally, the hachnosos kallah is to allow the kallah to fulfill her dowry requirements to her choson. Since you don’t have something doesn’t mean you should let the shidduch fall through since the kallah is unable to fulfill her dowry. This plain and simple basic Hachnosos Kallah.