I was asked by my Boss this afternoon - Thuhr time - to go to Jubail site (500 KM) away from Riyadh, thus I prayed - Thuhr and 'Asr prayers - in Jam' and Qasr before leaving. Later my Boss changed his mind and asked me to postpone my travel till tomorrow, and I didn't travel. My question is, should I pray Thuhr and 'Asr prayers again in 4-Raka'hs respectively or my prayers are valid since I was ready to travel the distance.

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. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If the reality is as you mentioned then repeating the Thuhr and 'Asr prayer becomes compulsory, since shortening the prayers before going out of the boundaries of the one's city is not allowed.  Allaah Says (which means): {And when you travel throughout the land, there is no blame upon you for shortening the prayer,…}[Quran 4:101]. So, shortening prayers before the journey is not allowed.

Anas Ibn Maalik, may Allah have mercy on him, narrated that the Prophet "The Messenger of Allaah would pray two Raka's if he had gone out a distance of three miles or Farsakh." [Muslim] So, the journey starts after actually leaving the boundaries of the village.

Allaah Knows best.