They could contribute to scientific progress by donating the money to a research university. Depending on your concept of immortality donating sperm or eggs seems to have a higher chance of success.
It's not a guaranteed win, no. However, if they are at the point where they wish to be euthanized then their outlook is grim. However, even a minuscule chance is better than zero chance, correct? In addition, by donating their brain structure they're directly fueling the research needed to improve that chance.