1) I made a vow to Allah, the Exalted, that I would contribute to the building of a mosque under construction with a certain sum of money if He blessed me with a boy. I was blessed with a boy, all praise is due to Allah, the Exalted. Is it permissible for me to offer that sum of money in two installments instead of offering it all at once?
2) I decided to buy a mus-haf (i.e. a copy of the Quran) whenever I received my salary and offer it to a mosque, but I realized later on that it will be more beneficial to offer the monetary value of the mus-haf to the mosques; can I do so?
Thank you.

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.

Whoever makes a vow to Allaah, the Exalted, to do a certain act of obedience to Him should deliver on his vow and do that very act. Hence, it is incumbent on you to give this sum of money, which you had vowed to give, to a mosque under construction all at once and not in installments if you can afford that. If you cannot afford it, then there is nothing wrong in paying it in installments, whether in two payments or more, according to your ability.

As for your second question, if this decision was a vow that you made by saying: "I make a vow to Allaah, the Exalted, to buy a Mus-haf for one of the mosques each month when I receive my salary," then you are obliged to deliver on this vow. It is impermissible for you to pay the monetary value of the Mus-haf to these mosques, unless they do not need the Mus-haf.

However, if it was merely an intention in your heart, or a mere decision that was not expressed in the form of a vow, then you are not obliged to deliver on this vow. However, the action you intended to do is considered an act of obedience that is encouraged in Islam. So, there is no wrong if you pay the monetary value of the Mus-haf to the mosque.

Allaah Knows best.