Monins living in western countries pay 40% tax from their earnings. Does this exonerate them from Khumas obligations or not. If not, don’t you think asking for 20% more from a person who has already paid 40% of his earnings an injustice to him?

No, it does not exonerate them from tax and giving to a government is different from giving to the imam and Islamic cause. The 20 % is only for the surplus. If a person does not have any surplus they don’t have to pay anything.