Sadaqah, according to the Quran, is a voluntary form of charity. In this case, the benefactor decides the amount offered.
On the other hand we have to refer to Zakat, which is actually a way of exercising 'sadaqah'. Zahat is the third of the five main pillars of Islam. It states, that not only is recommendable to practice charity, but that it is actually mandatory for all Muslims who enjoy a stable economic situation. A proportion of the global earnings of Muslim people have to be directed to aid those in need.