Responsa for Bava Batra 294:4

אי אמרת בשלמא דאורייתא משום הכי אינו יכול למחול אלא אי אמרת דרבנן היא אמאי אינו יכול למחול אינה של תורה ועשאוה כשל תורה

that [this is merely] Rabbinical, why should he not be able to remit [it]? — It is not Biblical; but was given<span class="x" onmousemove="('comment',' Lit., 'and they made it'. ');">¹²</span> [the same force] as [a law] of the Torah.<span class="x" onmousemove="('comment',' For the reason given supra, viz., lest his mind become affected. ');">¹³</span> Raba said in the name of R. Nahman: If a dying man said, 'Let X live<span class="x" onmousemove="('comment',' Lit., 'shall dwell'. ');">¹⁴</span> in this house', or,'Let X eat the fruit of this date-tree', his Instructions are to be disregarded<span class="x" onmousemove="('comment',' Lit., 'he said nothing'. X cannot acquire the right of living in the house or that of eating the dates. since the former is abstract, while the dates are nol yet in existence. As such rights cannot be given away by one in good health, even by means of symbolic and legal transfer, the acquisition of the object itself (the house or the tree) being required, a dying man also cannot by his mere verbal instructions (though valid in the acquisition of concrete and existing objects), confer such rights. ');">¹⁵</span>

Teshuvot Maharam