The idea that the donor can take his or her funds back (unless he or she dies or like this or a set time in the future passes), to make it more appealing to donate. So, it is kinda bequest to a charity.
How to say shortly and appealingly: "Donate a sum but allow me to take money back"?
Another idea is to have just "Donate" option but with also "Allow me to take money back before a date I set." checkbox. But this checkbox would appear just if the donate option is clicked, so somebody would not click it at all because he would not want to donate without the option to take back. That's bad, how to make it better?