The ruling on the fasting Person donating Blood
Question :
Is donating blood during the daytime in Ramadhan permissible or does it break the fast?
Answer:
If a person donates blood and a large amount of it is taken from him, this invalidates his fast based on an analogy with cupping. This is by blood being drawn from him through the veins to save a sick person or to preserve the blood for emergencies. However, if the amount that is taken from him is a small amount, then it does not break the fast. This is like the amount that is taken by way of needles and syringes to do blood tests and examinations.
Source:
Ash-Shaykh Ibn Jibreen
Fatawa Islamiyah, Vol. 3 Page 275