Article XXI. Section 219 of the present Constitution is hereby annulled and set aside and hereafter the legislature of Alabama may provide for the assessment, levy and collection of a tax upon inheritances and for the levying of estate taxes not to exceed in the aggregate the amounts which may by any law of the United States be allowed to be credited against or deducted from any similar tax upon inheritances or taxes on estates assessed or levied by the United States on the same subject. The legislature shall have the power to levy such inheritance or estate taxes in the state of Alabama only so long as and during the time an inheritance or estate tax is enforced by the United States against Alabama inheritances or estate, and shall only be exercised or enforced to the extent of absorbing the amount of any deduction or credit which may be permitted by the laws of the United States now existing or hereafter enacted to be claimed by reason thereof as deduction or credit against such similar tax of the United States applicable to Alabama inheritances or estates.