All obligations undertaken, whether prior or subsequent to the passing of this Act, by an institute or heir in possession of an entailed estate in Scotland, in any lease granted by him as proprietor of such estate, or in any agreement with reference to such lease, for the execution by the proprietor, or with reference to the execution by the tenant, of any improvements of the description contained in the third section of the [38 & 39 Vict. c. 61.] Entail Amendment (Scotland) Act, 1875, shall, in case of his death after the passing of this Act, and before complete fulfilment of such obligations, and to the extent to which, if he had himself made and paid for said improvements and had survived till payment is actually made, he would have been entitled to charge them upon the estate (if the estate had been an entailed estate under the said Act), devolve upon the heirs succeeding to the estate after him, who shall in their order be bound to relieve his executors, or other personal representatives, of such obligations, so far as unfulfilled, and to repay to such executors, or other personal representatives, any sums of money which they may be called upon to pay and may have paid in virtue of such obligations : Provided that this enactment shall not apply to any case in which the granter of the obligation has in express terms, either in the obligation itself or in any separate writing, declared his intention to impose the obligation upon his executors to the relief of his heirs of entail.