That's an interesting point actually. I'm not totally sure of the answers, but what I do know is that yes the dip can get sequestered into the fat in the wool of the animals, as farmers are advised to wash carefully after handling newly-dipped sheep for a few days afterwards. However, after a few days that risk is known to be diminished so presumably it moves back out of the wool and is washed away by rain, general rough and tumble. 

Another thing is that the type of thing organophosphates give rise to in humans are neurological... fatigue, dizziness... possibly it does have some effects on the sheep, but it would be difficult to tell. 

The sheep aren't prevented from ingesting it, but the design of sheep dipping baths is such that there isn't a great opportunity for ingesting the dip. They are only totally immersed for about 2 seconds. There's a bar below head level that they need to duck under to move on and out of the bath. 

All products granted a Veterinary Product Authorisation (that's what they're called in Ireland, there are similar in other countries), and intended for use in food animals, , have a specific withdrawal period worked out for them, which makes up part of the regulatory submission during the drug's development, and is printed on the container. There is one for milk and one for meat products. This takes the form of a statement "For the duration of treatment and X days afterward, products from this animal must not enter the human food chain" or "This animal must not be slaughtered for human consumption for a minimum of X days after the date of the last treatment".