is a Deviant because she has six toes.  David dreams that his father knows this and slaughters her in front of people in the same way that he does it to farm animals.  

My father held Sophie with one arm just as he had held the struggling calf. He raised his other hand high, and as he swept it down the knife flashed in the light of the rising sun, just as it had flashed when he cut the calf's throat....

The dream foreshadows that Joseph will eventually learn about Sophie's condition.  That's exactly what happens at the end of chapter five.  Joseph learns the specifics about Sophie because he beats David with a whip in order to force the information out of David.  

"Now he says this child you were with has six toes. Is that true?"

"No," I told him.

"He's lying," said my father. . . "I'll deal with this. The boy's lying." To me he added: "Go to your room."

I hesitated. I knew well enough what that meant, but I knew, too, that with my father in his present mood it would happen whether I told or not. I set my jaw, and turned to go. My father followed, picking up a whip from the table as he came. 

Well, the most obvious example of foreshadowing in this excellent dystopian novel is the very beginning, where David dreams of the city of Sealand that he finally reaches at the end of the novel. Note how it is described:

When I was quite small I would sometimes dream of a city - which was strange because it began before I even knew what a city was. But this city, clustered on the curve of a big blue bay, would come into my mind. I could see the streets, and the buildings that lined them, the waterfront, even boats in the harbour; yet waking, I had never seen the sea, or a boat...

Flicking to the end of the novel and the last section of the last chapter, David sees this sight in reality, rather than in his dreams. Note that he says:

It was just as I had seen it in my dreams... It was so familiar I almost misgave.

Wyndham therefore effectively foreshadows the happy ending and the safe arrival of David in Sealand by introducing this dream that becomes real at the end of the novel.

There are plenty of other examples that you can look at, especially the way that Sophie is used to show the fate of deviants and also the dream that David has of his father just about to sacrifice Sophie as if she were a mutant calf.

One example of foreshadowing is David’s father’s reaction to his innocent comment about having another hand.

David gets a splinter and tries to tie a bandage on his hand, but he is unable to do it.

'I could have managed it all right by myself if I'd had another hand.' (ch 3)

Although this is an innocent suggestion, David’s father becomes enraged.  He accuses David of asking to be a mutant and have three hands.

'You -- my own son -- were calling upon the Devil to give you another hand!' he accused me. (ch 3)

His father forces him to pray and beats him.  That night, he has a dream that his father slits Sophie’s throat like the baby calf.  This dream is prophetic because David’s father later beats forces him to tell about Sophie.

As a great novel, this text has many examples of foreshadowing, however you have highlighted an excellent example in the dream that David has, which foreshadows the ending of the novel and his arrival at Sealand. Another example that is very poignant comes in Chapter 7. David's mother's sister, Aunt Harriet, comes to visit with her new child, just as David's mother has just had Petra. However, it is clear that unfortunately this child, like the two others she has had, is classed as a "deviant". Harriet makes it clear what kind of position this leaves her in:

"This is the third time. They'll take my baby away again like they took the others. I can't stand that - not again. Henry will turn me out, I think. He'll find another wife, who can give him proper children. There'll be nothing - nothing in the world for me - nothing. I came here hoping against hope for sympathy and help. Emily is the only person who can help me. I - I can see now how foolish I was to hope at all..."

What this event foreshadows is the hardness of David's father and his mother in their harsh obedience to the purity laws - even at the cost of turning away family to an uncertain fate. Just as they turned away and rejected Harriet, so too will they turn away and reject their own son and daughter. We learn at the end of the chapter that Harriet killed herself, and presumably the baby, out of despair. This of course paints a terrible future for anyone who is either deviant or trying to hide a deviant individual.

If you want more examples of foreshadowing, you might want to consider David's dream about his father sacrificing Sophie as he sacrifices beasts. Hope this helps! Good luck.