What does Genesis 20:16 mean?
Years earlier, the Lord had promised to unconditionally bless Abraham. He continues to do so here. Even when Abraham appears to act dishonorably in fear and faithlessness, God blesses him in it. It happened after Abraham's lie about Sarah being his sister in Egypt, and it happens again here.King Abimelech, eager to be healed and show himself obedient to Abraham's God, has returned Sarah, untouched (Genesis 20:4), to Abraham along with great gifts: herds, cattle, servants, and the choice of any land in the kingdom for a homestead (Genesis 20:15). Now the king goes even further, blessing Abraham and Sarah in two more ways.
First, Abimelech gives Abraham a thousand pieces of silver. That's a lot of money in this era, far beyond the normal price for a bride. Abimelech's stated reason for doing so is the second blessing: It is meant as a sign of Sarah's innocence in this matter. Somehow, this large sum of silver was meant to protect Sarah's reputation, to tell everyone that she had not been compromised while in Abimelech's household.
Notice that while telling this to Sarah, he still calls Abraham her "brother" and not her husband. Since that was the half-truth, half-lie used by Abraham, Abimelech might mean that statement as a not-so-subtle sign of his continued displeasure with fearful Abraham.