The amount of interest acquired must be returned to wherever you received interest from. If that is not possible, the interest amount must be given in charity (but not as zakat). This is a debt and liability upon you. Hence, when calculating Zakat, interest is a deductible from your gross Zakatable assets.
Money acquired unlawfully belongs lawfully to the owners of the wealth. If they cannot be traced, one must dispense of such wealth in charity as that amount is not lawfully yours. Therefore, unlawful wealth is a liability which must be dispensed and will be deductible from one’s Zakatable assets for the missed year.
And Allah knows best!
Reviewed on 29/11/2021