Question: Is it permissible for the Imām to make a collective dua after the Fard Salāh?
Answer: Although congregational Dua after Farḍ Ṣalāh is not explicitly proven from any Ḥadīth, this much is proven that it is a time when Duas are accepted, also the virtues of collective Dua have been proven in many ‘Aḥādīth, and since here the congregation is automatically formed due to Ṣalāh, therefore there is no harm in making a collective Dua occasionally and it would be incorrect to regard it as Bidʿah and innovation. This is unlike Dua after Sunnah or Nafl Ṣalāh as this is performed individually and not in congregation, and also that Dua after Farḍ Ṣalāh has been proven, not after Sunnah and Nafl Ṣalāh.
Furthermore, this method should not be regarded as being compulsory and should this impression begin to prevail, the best way to eradicate it would be to alternate between collective Dua and individual Dua. This is the best way to educate the people about the correct position.
Also Dua should not be made in a very loud voice whereby it becomes the source of disturbing the Salāh of those who joined the congregation after it had started and are busy completing the missed rakats after the Imām has finished. Therefore a reasonable audible voice is best.
The above mentioned ruling has been derived from the following books of Fiqh:
Fatāwā Darul ʿUloom Deoband vol.2 pg.242
Anāfaisul Marghūbah Fī Ḥukmi Duʿai Baʿdal Maktūbah: pg 65.
Fatāwā Darul ʿUloom Zakariyya: vol.1 pg. 185-191; vol. 4 pg. 707-714
And Almighty Allah Knows Best