What is the ruling on executing a bequest (wasiyyah) when the heirs approve it?

Primary text

If a person bequeaths that Zayd performs Hajj on his behalf for one hundred, and Amr receives the remainder of one-third, and Sa'd receives one-third of the estate, the bequest must be executed as the testator specified if the heirs grant approval. If, after allocating one hundred to Zayd, nothing remains from the one-third share designated for Amr, then Amr receives nothing because his share was contingent upon the remainder, and no remainder exists.