A waqf does not become binding by merely forming the intention

A waqf does not become binding by merely forming the
intention that something should be a waqf. Rather he has to utter words to
that effect, such as saying, “This is a waqf”, or do an action to indicate
that it is a waqf, such as if he builds a place then gives the call to
prayer (adhaan) in it, which indicates that he is giving it as a waqf to be
a mosque.

It says in al-Mawsoo‘ah al-Fiqhiyyah (44/119-120)

The fuqaha’ differed concerning how binding it is when
something is designated as a waqf, and if it can be retracted. The majority
of Maaliki, Shaafa‘i and Hanbali fuqaha’, as well as Yoosuf and Muhammad
among the Hanafis, are of the view that once a waqf is given by someone who
has the authority to dispose of wealth and fulfils all the necessary
conditions, then it becomes binding and cannot be retracted, and the one who
made it a waqf no longer has any authority over the property given as a waqf
in any way that undermines the purpose for which it was made a waqf. So it
cannot be sold, given as a gift or inherited. That is because the Prophet
(blessings and peace of Allah be upon him) said to ‘Umar ibn al-Khattab (may
Allah be pleased with him): “Give it in charity as a waqf, not to be sold or
given as a gift or inherited.” Because the waqf is a donation that prevents
it from being sold, given away for inherited, it immediately becomes binding
(and cannot be retracted) as soon as the donor utters words to that effect –
as in the case of manumission of a slave. It is unlike giving a gift,
because giving a gift means transferring ownership completely, whereas in
the case of a waqf, the property itself is kept, but the benefit is given
away as charity. Hence it is more akin to manumission and it is more
appropriate to draw an analogy with it.

According to Abu Haneefah, if someone makes a waqf, it is
permissible to retract it and it is not binding for ever. He may retract it
within his lifetime, although that is makrooh, and it may be inherited from
him. … But the view that became the official Hanafi view is based on the
view of Abu Yoosuf and Muhammad, which is that it is binding and cannot be
retracted. Ibn ‘Aabideen said, quoting from al-Fath: The correct view
is the view of the majority of scholars, that it is binding and cannot be
retracted, because there are many hadiths and reports to that effect. That
was the view followed by the Sahaabah and Taabi‘een and those who followed
them, and their view is more likely to be correct. End quote.

Ibn Qudaamah (may Allah have mercy on him) said:

Ownership ceases and the waqf becomes binding (and cannot be
retracted) as soon as words to that effect are uttered, because the waqf
takes effect as soon as the words are spoken. There is another report from
Ahmad (may Allah have mercy on him), according to which it does not become
binding unless it is received by the caretaker and the donor hands it over.…
Then he quoted evidence for the validity of the first view, then he said: We
have proof in what we have narrated of the hadith of ‘Umar, because it is a
donation that disallows selling, giving away and inheriting, so it becomes
binding by merely uttering the words, as in the case of manumission. This is
in contrast to giving a gift, because giving transfers ownership in full,
whereas in the case of a waqf, the original property is kept, and the
benefit is given to specific recipients. Hence it is more akin to
manumission and it is more appropriate to draw an analogy with it. End
quote.

al-Mughni
(6/ 208)

Based on that:

If the owner of the house merely had the intention (and no
more) of making the house a waqf for the sake of Allah, may He be exalted,
to be attached to the mosque, then he is not obliged to go ahead with that;
rather he may retract his intention. But if he clearly stated that it was a
waqf after buying the house, and he said: “It is a waqf for the sake of
Allah, may He be exalted,” then in that case it is not permissible to
retract it. Rather he must go ahead and make it a waqf as he said, and if he
promised the people of the mosque that he would make the house a waqf for
the sake of Allah, may He be exalted, to be attached to the mosque, then it
is better for him to fulfil his promise, for that is one of the most sublime
of Muslim characteristics.

The scholars of the Standing Committee for Issuing Fatwas
were asked:

His Excellency Prince ‘Abd ar-Rahmaan ibn ‘Abdullah Aal Sa‘ud
promised to give a piece of land near the village of ad-Dubay‘ah to build a
school, but doing that depends on whether it is permissible to go back on a
previous promise to give it to build an Eid prayer place.

 His Excellency asked us to consult the scholars concerning
that, and whether he should choose to give it to build an Eid prayer place,
in fulfilment of the previous promise, or to give it to the Ministry of
Education to build a school on it? Please note that there is already a
mosque for Eid prayer in the west of ad-Dubay‘ah.

They replied:

If His Excellency Prince ‘Abd ar-Rahmaan ibn ‘Abdullah Aal
Sa‘ud has already given the piece of land to build an Eid prayer place on
it, then it is to be used to build an Eid prayer place, and he has no right
to retract his gift. If what happened was merely a promise to give the land
to build the mosque on it, then it is better for him to fulfil his promise.
End quote.

Shaykh ‘Abd al-‘Azeez ibn Baaz, Shaykh ‘Abd ar-Razaaq
‘Afeefi, Shaykh ‘Abdullah ibn Ghadyaan

Fataawa al-Lajnah ad-Daa’imah
(16/93, 94)

See also the answer to question no.
125101

And Allah knows best.