The grapes are one of the crops that are measured and saved, so Zakah is due on it when it reaches the Nisab (the minimum amount on which Zakah is due), which is five Wasqs (1 Wasq = 60 Sa`s = 180 kg. Approx.) that is equal to 300 Sa`s (1 Sa` = 3 kg. Approx.) using the Sa` of the Prophet (peace be upon him). Thus, grapes should be estimated when they show signs of ripening like the estimation of the ripening of palm trees; and the Zakah should be given from raisins in the same manner it is given from dry dates in the case of palm trees. The amount of Zakah is 10% on the land that is naturally irrigated without incurring any costs, and only 5% on the land irrigated by machinery that incurs costs. But if the land is irrigated by both manners equally, then the Zakah is 7.5% of the production; and if the ways of irrigation differs from each other, then consideration will be given to the prevailing case, i.e. if mostly irrigated by machinery, only 5% is due; otherwise, 10% is due.As for what you give of the fruits to people who ask for it, or to those in their homes during the harvest, it is acceptable as Zakah on two conditions: First: the recipients are poor. Second: you give it to them intending it as Zakah and knowing its amount. If these two conditions are fulfilled, it may be calculated from the Zakah. (Part No. 8; Page No. 57) However, if the people who receive the grapes are not of the legal recipients specified or if you give them with no intention to be a part of the due Zakah, it is a form of Sadaqah (voluntary charity) that is rewarded, in sha’a-Allah (if Allah wills).The debt due on you that surpasses the price of the harvest does not prevent the obligation of Zakah that is due on you, for the generality of the Ayah (Qur'anic verse) where Allah (Glorified be Allah) states:
...pay the due thereof (its Zakât, according to Allâh’s Orders 1/10th or 1/20th) on the day of its harvest
Allah (Exalted be He) also states:
O you who believe! Spend of the good things which you have (legally) earned, and of that which We have produced from the earth for you
And perform As-Salât (Iqâmat-as-Salât), and give Zakât and obey the Messenger (Muhammad صلى الله عليه وسلم) that you may receive mercy (from Allâh).
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.