This isn't really specific to the special case of having negative money. If you have $10,000 to give and can invest that in the stock market and earn money at a rate higher than your discount rate for lives, then you should invest the money in the stock market and give to charity in a years time. if this continues to be the case, you should continue to compound your money and will it all to charity when you die.

Now if someone can just give me a good way of figuring out what my discount rate for lives is, this would be helpful in answering this question for myself.