Is zakat due on the accumulated salary of a detained (imprisoned) person? How is it paid? Is it divided? And is it due for one year but not another?

Zakat is obligatory on saved money if a [lunar] year has passed on it and it has reached the nisab (minimum threshold). Zakat must be paid for past years in which the conditions for its obligation were met. The amount can be determined by bank statements, or by diligent estimation and investigation to the best of one's ability if that is not possible, as Allah Almighty says: "Allah does not charge a soul except [with that within] its capacity" (Quran 2:286). The money spent on expenses is not counted as part of the balance on which zakat is due. Zakat is distributed to its [designated] recipients and is calculated for each year separately. The nisab for banknotes is approximately equivalent to 85 grams of gold or 595 grams of silver, and one-quarter of one-tenth (2.5%) must be paid.