I live in a European country. I made a vow to Allah, the Exalted, that I will buy a Mus-haf (copy of the Quran) and put it in the mosque, but the imam told me that the Saudi Embassy provides the mosque with a sufficient number of Mus-hafs and that they are in no need of more. My question is: can I offer the monetary value of the (vowed) Mus-haf instead, especially given that the mosque is in dire need of money?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

According to the basic rule, it is obligatory for the vow-maker to fulfill his vow exactly as he had vowed. It is impermissible for him to offer any substitute for the vowed object as long as he is able to offer it itself. This is the opinion of the majority of the Islamic jurists. Therefore, it is incumbent on the questioner to deliver on his vow even if the mosque has enough Mus-hafs. This is because the Mus-haf donated by you is surely to be used some times. However, some scholars view that it is permissible for the vow-maker to offer the monetary value of the vowed object. Based on this view, there is nothing wrong if you pay the monetary value of the vowed object to the mosque, especially if the mosque is in need of money to make repairs, for lighting, or for any other service that is needed by the worshipers and there are enough Mus-hafs. The opinion of the Hanafi scholars in this regard, as stated in Multaqa Al-Abhur, is, "It is permissible to pay the monetary value of the Zakah, ‘Ushr (i.e. one tenth of land produce which is due as Zakah on crops), Kharaj (i.e. land-tax), expiations, vows, and Zakatul-Fitr."

According to Al-Mawsoo‘ah Al-Fiqhiyyah (The Encyclopedia of Islamic Fiqh):

It is permissible to opt for offering a substitute for the vowed object as long as offering the substitute constitutes a manifest sharia-approved interest. Some Hanafi scholars considered it permissible to pay the monetary value of the Zakah, charity, Zakah-al-Fitr, vows, and expiations.

Allaah Knows best.