He gave his mother a gift then she bequeathed it to him after her death

Question

Praise be to Allah.

What you gave to your mother and she accepted from you has
become her property, and the basic principle is that it is part of her
estate which is to be divided among all of her heirs. But her saying that
gifts are to be returned to the one who gave them after she died, which is
clearly a bequest in your favour, is a bequest to an heir as you say. Abu
Dawood (2870), al-Tirmidhi (2120), al-Nasaa’i (4641) and Ibn Majaah (2713)
narrated that Abu Umaamah said: I heard the Messenger of Allaah (peace
and blessings of Allaah be upon him) say: “Allaah has given each person who
has rights his rights, and there is no bequest for an heir.” Classed as
saheeh by al-Albaani in Saheeh Abi Dawood.

This bequest should not be executed unless the heirs agree,
so long as they are adults of sound mind. As for one who is not an adult of
sound mind, such as a minor, his agreement does not count, and it is not
permissible to detract anything from his share in the interests of the one
for whom the bequest was made.

It says in one of the versions of the hadeeth quoted above
from Ibn ‘Abbaas (may Allaah be pleased with him): “It is not permissible to
make a bequest to an heir unless the other heirs agree.” Narrated by
al-Daaraqutni and classed as hasan by al-Haafiz Ibn Hajar in Buloogh
al-Maraam.

Ibn Qudaamah (may Allaah have mercy on him) said in
al-Mughni (6/58): “There is no bequest for an heir, unless the other
heirs allow that.” To sum up, if a person makes a bequest to one of his
heirs and the other heirs do not agree to it, then it is not valid, and
there is no difference of opinion among the scholars concerning that. Ibn
al-Mundhir and Ibn ‘Abd al-Barr said: The scholars are unanimously agreed on
that, and there are reports from the Messenger of Allaah (peace and
blessings of Allaah be upon him) to that effect. Abu Umaamah narrated: I
heard the Messenger of Allaah (peace and blessings of Allaah be upon
him): “Allaah has given each person who has rights his rights, and there is
no bequest for an heir.” Narrated by Abu Dawood, Ibn Majaah and al-Tirmidhi.
… But if they agree to it then it is permissible, according to the majority
of scholars. End quote.

It says in Fataawa al-Lajnah al-Daa’imah (16/317): It
is not permissible to bequeath more than one-third, and it is not valid to
make a bequest to an heir, unless the heirs who are adults of sound mind
give up part of their shares, because the Prophet (peace and blessings
of Allaah be upon him) said: “Allaah has given each person who has rights
his rights, and there is no bequest for an heir.” Narrated by Ahmad, Abu
Dawood, al-Tirmidhi and Ibn Majaah, and by al-Daaraqutni who added at the
end: “unless the other heirs agree.” End quote.

And Allaah knows best.