Fergus didn't offer only to retreat in battle himself. He promised to retreat with the men of Erin in tow.
From 19. The Battle Of Fergus And Cuchulain (emphasis mine)
"...give way before me this day in the presence of the men of Erin!" "Truly I am loath to do that," answered Cuchulain, "to flee before any one man on the Cattle-spoil of Cualnge." "Nay then it is not a thing to be taken amiss by thee," said Fergus; "for I in my turn will retreat before thee when thou wilt be covered with wounds and dripping with gore and pierced with holes in the battle of the Táin. And when I alone shall turn in flight before thee, so will all the men of Erin also flee before thee in like manner."
So zealous was Cuchulain to do whatever made for Ulster's weal that he had his chariot brought to him, and he mounted his chariot and he went in confusion and flight from Fergus in the presence of the men of Erin.
So Fergus has not just offered Cuchulainn an out on a difficult situation, he's offered him a serious advantage on the battlefield, with which to defend Ulster.
Cuchulainn makes clear with his initial rejection of the agreement that simply not wanting to kill (or be killed by) his foster-father is not enough to compel him to yield. Only something gained "for Ulster's weal" will suffice.
Ferdiad, on the other hand, had only just arrived (against his will) in Mebd and Ailill's camp. He had no armies with which he could offer such an advantage to Cuchulainn. Even if he had wished to make such an agreement, he would not have been able to offer much, apart from his own life, which was already being offered in single combat.