1590. If a person observing fast becomes so thirsty that he fears that he may die of thirst or sustain some harm or extreme hardship, he can drink as much water as would ensure that the fear is averted. However, his fast becomes invalid, and if it is the month of Ramadhan, as an obligatory precaution, he should not drink more than that, and then for the rest of the day, refrain from all acts which would invalidate the fast.

Therefore there is no Kaffara