You claim that the benefits you could give tomorrow "would be greater" but what's the argument? The argument is that tomorrow you would have "$258.3 million"?
Let's assume you can double your money over the course of 7 years. AMF (Against Malaria Foundation) protects whole villages with anti-malaria bed nets, often resulting in a drop of around 50% of incidences of malaria over the course of a year or two. This also results in preventing some deaths.
While you wait your 7 years to provide double the money to this charity, many people will needlessly die. And with higher prevalence of malaria, more people will be infected than you can prevent later.
In your example you compare Bill Gates's donation to malaria eradication with giving money to the homeless. How is this the right comparison? And also, bringing Bill Gates into the discussion is irrelevant, unless you're in possession of MILLIONS of dollars you are planning to donate. Are you?