If the person lives in that house and it is not an investment property, they do not have to pay khumus for the extra money that they are paying the mortgage with, because paying of a loan is part of our  yearly expenditure and therefore it is not part of khumus. Also if a person has taken a loan for daily living in that financial year, than at the end of the financial religious year even if that person has not payed the loan off they can see how much money they have in their possession and take the same amount that they owe out of the income they have and than calculate the khumus they must pay. For example, if one borrows $100000 of daily living not for invest nor business and at the end of the financial year one had $10000 left over, they do not have to pay khumus because they borrowed $10000 for my daily living. However, if one has borrowed a similar $10000 for daily living expenditure but from previous years, e..g 2-3 years prior, and want to calculate how much khumus they must pay; If they did pay this $10000 before the end of the religious financial year than one does not have to pay khumus for that $10000. Now if that $10000 has not been payed back by the end of the  financial year, they must pay  khumus for that amount. That is why if a person who has a mortgage and does not want to pay khumus for any extra money left over, they should put this money into their mortgage. Also if the loan taken from overseas the repayment has been made but has not reached the lender before then end of the religious financial year than khumus must be taken.