What if a believer who is an employed worker, has prayer times colliding within his working hours for example Maghrib Salaah. In extenuating circumstances due to work becoming busy what is the very latest time he can afford to pray until OR as soon as the sky is completely sun set is Maghrib Salaah no longer permissible for that particular day?

Al-jawaab billaahi at-taufeeq (the answer with Allah Taa’la's guidance)

There are no justifiable (extenuating) circumstances or allowances for missing a Salaah; for this reason, the Ulama say a person cannot work in a place which doesn't provide you with provision to offer your Salaah on time. Hadhrat Abdullah Ibn Amr (Radi Allah Anho) said that the Holy Prophet, Hadhrat Muhammad-ur Rasullullaah (Sallallaahu Alayhi Wasallam) said, "The time for Magrhib continues until the twilight disappears”. On the bases of the previous Hadith, this is the conclusion what the scholars have agreed upon unanimously. You will see that for this reason the Hanafi school of thought normally prays Maghrib immediately after sunset. I would say that as a result we should pray Maghrib just after the sunset and should not delay it for more than an hour after sunset.

 (VERILY, ALLAH TAA’LA IS ALL-KNOWING!)