Merely because assessee was following mercantile system of accounting, it could not be held that income had accrued to it. Earning of the income, whether actual or notional, has to be seen from the viewpoint of a prudent assessee. If in given facts and circumstances the assessee decides not to charge interest in order to safeguard the principal amount and ensure its recovery, it cannot be said that he has acted in a manner in which no reasonable person can act