Some questions.
How many eggs are were concerned about here.? 10,, 100,,1,000 ?
If the number is low, then just toss them.
If you do feed then back to your chickens, the withdraw period stretches then beyond the normal, period of (from last day of treatment) (if you keep giving them their eggs)
Hard to say how long the withdraw period should be then. You would be feeding them somewhat potentially contaminated eggs with fenbendazole.
I don't have a definite answer, and would prefer to error on the safe side myself. I would toss all the eggs as per the recommended withdraw period.
If you really do want to feed them back to your chickens, choose those not laying eggs,, or roosters.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,and