Generally speaking, for a bequest to be valid, a Legatee must be in existence at the time of death of the Testator except in the case of a general and continuing legatee such as the poor, orphans etc.
The Tegatee must be capable of owning the bequest. Any bequest made in favor of any legal heir already entitled to a share is invalid under traditional Sunni Muslim law unless consented to by other legal heirs. An acknowledgement of debt in favor of a legal heir is valid.
Acceptance or rejection of a bequest by the Legatee is only relevant after the death of the testator and not before. Generally speaking once a Legatee has accepted or rejected a bequest he cannot change his mind subsequently.
If the Legatee dies without accepting or rejecting the bequest, the bequest becomes part of the Legatee’s estate according to the Hanafi fiqh because non-rejection is regarded as acceptance. According to the other three main Sunni madhahib, the right to accept or reject the bequest passes onto the heirs of the Legatee.
There is difference of opinion as to the time at which ownership of a bequest is transferred from the Testator (or his heirs) to the Legatee. According to the Hanafi and Shafii fiqh the transfer of ownership is at the time of death of the Testator, according to the Maliki and Hanbali fiqh the transfer of ownership is at the time of accepting the bequest.
All the Sunni madhahib agree that if the Legatee dies before the testator, the bequest is invalid since a bequest can only be accepted after the death of the Testator.
If there is uncertainty as to whether or not the Legatee survived the Testator, such as a missing legatee, the bequest is invalid because the Legatee must be alive at the time of death of the Testator for the will to be valid.
If the Testator and Legatee die together, such as in an air crash, and it is not certain who died first, the bequest is invalid according to the Hanafi, Maliki and Shafii fiqh. But according to the Hanbali fiqh, the bequest devolves upon the Legatee’s heirs who may accept or reject it.