All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If the questioner means that she had vowed to give a certain sum of money to the poor in charity if Allaah, the Exalted, blessed her with recovery from her illness and she actually recovered from her illness, then it is incumbent on her to offer that sum of money in charity, exactly as she had vowed. It is impermissible for her to offer any substitute for the vowed object. This means that she has to give it in the form of money; not food or anything else. Nevertheless, some scholars held the view that it is permissible for the vow-maker to offer the monetary value of the vowed object. The Hanafi scholar Ibn Nujaym said, "When the person vows to give an item of wealth in charity but offers an equivalent substitute for the vowed object or its monetary value, then there are two opinions in this regard." [Al-Bahr Ar-Ra’iq Sharh Kanz Ar-Raqa’iq]
If the charitable society to which she had paid the vowed money is reliable and trustworthy and she told the officials that she had vowed to offer that money and they accepted it based on this information, then this is considered sufficient for her and she has fulfilled her vow, even if the coupons she bought were for offering Iftaar meals for fasting people. This is because she has delegated this charitable society to spend the vowed money as charity to the poor. They should be trusted as long as they are trustworthy and reliable. The same applies for Zakah. This is of course unless her giving those coupons (for fast-breaking meals) means that they will spend the vowed money in the form of food. In such a case, it will not clear her liability according to the scholars who adopted the view that the vow-maker is obliged to give the vowed object itself.
If her intention while making the vow was merely to give charity to the poor with money or anything else, then it will be sufficient for her to offer food or anything else. An-Nawawi said in Al-Majmoo‘, a book on the Shaafi‘i school of fiqh, "If the vow-maker says, 'If Allaah, the Exalted, blessed my patient with recovery from his illness, I vow to offer something in charity,' then his vow is valid and binding. If he gives anything in charity, great or small, it will be sufficient for him."
However, if she gave the vowed money to the poor herself, then that would be better so as to make sure that the vowed money is given to its rightful recipients. Moreover, the acceptance of one's deeds is a matter of the unseen that only Allaah, the Exalted, knows. However, if the due conditions of the validity of an act are fulfilled, then it is more likely to be accepted by Allaah, the Exalted. Among those conditions are offering this charity for the sake of Allaah, the Exalted, seeking His pleasure, and that the money is lawful and pure; Allaah, the Exalted, will not allow the reward of the doers of good to be lost. Although the acceptance of the deeds by Allaah, the Exalted, is a matter of the unseen, the believer is asked to strive to have his deeds accepted by Allaah, the Exalted. He should utilize all the available means to attain such an objective. We implore Allaah, the Exalted, to accept our and your righteous deeds.
Allaah Knows best.