Is the amount of donations a right of the daughter, or should it be returned to the donors? And should the amount obtained from selling the land be used to complete the daughter's treatment, or to start a business?

If the funds were given as charity for your daughter's treatment, they are her property, even if the chances of recovery are slim; because doctors' prognoses are not definitive, and seeking medical treatment is permissible. Your acquiring wealth does not obligate you to return these funds, nor does it compel you to treat her with your own money as long as what was donated for her is sufficient. You are free to dispose of your private funds in permissible ways.