Are the funds given after answering an exam result, when there is a failing grade, considered permissible or impermissible, and what should I do if they are impermissible?

The indirect speech (ma'arid) you used to take your grandmother's money is an illegitimate trick, because through it you obtained money you do not deserve. Indirect speech becomes unlawful (haram) if it is used to acquire something false, or to repel a right, or if its intent is to conceal something that ought to be made manifest, such as a testimony or an admission. You must return the money to your grandmother or seek her forgiveness, and repent from the lying and the unlawful indirect speech.