Some scholars do not allow zero percent finance schemes. If you have gone ahead with the opinion of those who allow it, if the contract is of shorter term like 12 months some scholar say you can deduct. The opinion of Shaykh Abu Eesa is that installments amount cannot be deducted from the zakaat that you need to pay.
You have to strictly pay zakaat on the money you have on the hijrah date of zakaat once a year, you cannot deduct future payments from the amount of money you have right now and pay zakaat on that.
Shaykh Abu Eesa explains and answers...
I have purchased a car using 0% financing, and I'm paying it off every month. Can I use these payments, and deduct them from what I would pay as Zakaat?
Couple of things, straight off the bat. Some scholars don't allow the concept of 0% financing, if you have to sign an agreement that says that you would pay interest if you do not make the payment. I am actually one of those. But as a side point, because a number of scholars do allow 0%. So, assuming that the 0% financing is allowed for that purchase, the second, then, issue is zakaat itself. There are some scholars, for example, that say that if you're in a short-term kind of contract, then 12 months of liability that you would pay, ie, debts that you would pay, can be deducted from the amount of zakaat that you need to pay when your date comes 'round. I am definitely not of that opinion.
I just want you to be aware, for the academic completion of this answer, but you are asking me, I'm going to give you the answer which I strongly believe. I do not believe that a person should deduct anything from their sum of money that they have on them on the date that comes 'round, the Hijri date that they have to pay once a year. So, if you choose that date to be the first of Ramadan, many folks do choose Ramadan. But, let's say if you're a better man than that, and you choose Shabaan, you've let Ramadan be a sadaqa month, but you do your obligatory zakaat at another time of the year, so that people don't have to wait for Ramadan until they are saved.
The first of Shabaan comes 'round, and you've got 10,000 pounds. You will pay zakaat on that 10,000 pounds. It doesn't matter whether you owe 5000 pounds, and it doesn't even matter if you're owed 5000 pounds. If I don't have that extra money here with me right now, I'm not paying on it, and I don't care if I owe someone else that money, if I've still got their money, then it's still mine, it's still my liability, I am paying upon it. So, me, I do not allow any deductions, monthly deductions, anything. If I paid my monthly installment yesterday, and today I've got 9500 pounds, I pay on 9500 pounds. If my monthly installment is going out tomorrow, so it's going to be 9500 pounds tomorrow, but my Hijri date lands today, and it's 10,000 pounds, I'm paying on 10,000 pounds.
If a person I owe money to, they come and take their money tomorrow, and it drops to 5000 pounds, but today it's 10,000, I'm paying on 10,000 pounds. If a person who owes me money, and then brings me that money tomorrow, and it's 15,000 pounds that I should have paid Zakaat upon tomorrow, but today it's 10,000, I pay on 10,000. That is the easiest way of paying zakaat. There can be no one that can criticize this. All the fatawa that talk about debt, strong debt, weak debt, apply it, whatever. As scholars that are trying their best to help you, and try to get around the modern financial markets, et cetera, and there are some opinions that support this as well. But with me, I'm very, very clear upon this. You pay upon what you have, and that's it. Allah knows best.