Example: Cash as of YR 1 Khums date is 20,000; Khums paid 4,000; Post Khums (Mukhammas) balance 16,000 YR 2 closing cash balance is 19,000. In my opinion, there could be two ways of calculating Khums at the end of YR 2 19000-20000 = No khums - net cash has decreased overall, so there should be no Khums. and this makes sure that there is no need to record a cumulative Mukhammas balance. Or 19,000-16,000=3,000 (khums) This means Khums of 4,000 is not considered to be an admissible expense of YR 2.
The time of calculation of Khums is the end of the year of Khums which starts when you started earning. Any wealth more than last year's Mukhammas ( post Khums paid remaining amount) should be calculated for Khum of this year on the date of Khums.