Answered by: Maulana Abrarul H. Hasib
Respected Mufti Sb
If a person makes wasiyat; the legatees are ‘grandchildren’ and the amount is ‘remainder of one third’ for ‘education of hafiz and Aalim only’ (the person had made another wasiyat with a specific amount thereafter mentioned this one).
At the time of death the deceased had a few grandchildren out of which only one had begun hafiz classes thereafter other grandchildren started Hifz and others were born too. The deceased also have children that are not yet married.
Is the above wasiyyat correct, for whom can the funds be used; all grandchildren that will be born or only those who were alive at the time of death or only the one doing hifz? must/can it be kept for those to be born after the passing on of the deceased and if there is any excess can it be carried over to the greatgrandchildren?
Can the executor of the estate, after evaluating the entire estate, take out the 1/3 wasiyat in the form of cash only -making it easy to fulfil the wasiyat and leaving the remaining cash and other assets with the inheritors as per shari division
Can the funds be invested? can it be borrowed from someone? Can the wasi use it then replace it? Will the funds remain in the custody of the wasi or will it go to the father of the child?
In the name of Allah, the Most Gracious, the Most Merciful
First and foremost, one of the conditions of Wasiyyat (bequest) is that the legatees are not Waaris (inheritor) himself/herself, this bequest will be correct if the grandchildren are not Wariseen. 
This bequest will be applicable to those grandchildren who were present (who were alive physically or who are in the womb of a mother) on the day of his (Musi) death. 
Therefore, those future grandchildren who weren’t conceived in the womb during his death, will not be included in that wasiyyat and bequest.
As far as the executor of will taking out in the form of cash and fulfil wasiyat is concerned, it will be permissible with the pleasure of every inheritor. 
After funeral expenses and debts are settled, bequest and wasiyyat will be resolved to the legatees or their walis (guardian). 
Wali and the guardian are allowed to invest and do business with his wealth. 
Only Allah knows best
Written by Maulana Abrarul H. Hasib
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 “وشرائطها كون الموصي اهلا للتمليك) وكون الموصى له حيا وقتها تحقيقا أو تقديرا ليشمل الحمل الموصى له وكونه غير وارث وقت الموت” (الدر المختار مع الرد المحتار: ج: 10، ص: 337، دار عالم الكتب)
 Fatawa Qasimiyyah, Vol. 25, Pg. 54 & 57, Maktabah Ashrafiyah.
 “ثم تنفذ وصاياه من ثلث ما بقي بعد الدين”. (السراجي في الميراث: ص: 6، مكتبة البشرى)
 ولا يتجر الوصي في ماله اي اليتيم لنفسه فإن فعل تصدق بالربح وجاز لو أأجر من مال اليتيم لليتيم. (الدر المختار مع الرد المحتار: ج: 10، ص: 425، دار عالم الكتب)