In the event that one of the States ceases to permit residents of the other State who are liable to tax in the first-mentioned State on income from immovable property situated therein to compute, in accordance with and subject to the domestic legislation of the first-mentioned State, the tax on such income on a net basis, then a resident of the other State who is so liable to tax on such income may compute the tax on such income in accordance with and subject to the domestic legislation of the first-mentioned State as it read at the time that State ceased to permit the computation. Interest on any mortgage or other indebtedness secured by immovable property situated in one of the States shall be deemed to arise in that State to the extent that, pursuant to any computation referred to in the preceding sentence, such interest is deducted in computing the tax on income of a resident of the other State from immovable property situated in the first-mentioned State.
