Dought & clear,- He promised his mother to give her money that she could put towards building a mosque, then she died.

My mother asked me for some money that she could put towards a mosque
that was being built in our village, but at that time I did not have
any money that was surplus to my needs with which I could fulfil her
request. So I promised her that in the future I would give her money
if Allaah willed. Then I travelled outside of my country to seek
halaal provision. During that period, Allaah decreed that my mother
fell sick and she died as a result. Does that promise that I made to
her remain as a debt that I owe, and do I haveto give what I promised
her to that particular mosque, or is it permissible to give it to
another mosque that needs money, or is that promise no longer a debt
that I owe and am Ithus pardoned from it?.
Praise be to Allaah.
Yes, you must certainly fulfil your promise and give that money to the
mosque that your mother wanted to give itto, as that you promised to
do. It is part of honouring her and treating her kindly to fulfil this
promise, in the hope that Allaah, may Hebe glorified and exalted,will
accept its and benefit her thereby. Thisis part of honouring her and
treating her kindly. If the mosque that your mother wanted to support
has no need of it and its expenses are covered, then you may give it
to any other mosque, but as we have mentioned, you should not neglect
this promise and fail to give this money, rather you should and most
certainly must fulfil this promise, as a way of honouring your mother.
And Allaah knows best. End quote.
Majmoo' Fataawa al-Shaykh Saalih al-Fawzaan (2/708).