Pharaoh took Soroi and benefited Abram for her sake 12 (16).
He did this on the understanding that Soroi was Abram's sister. He asked 12 (18)
“Why did you not tell me that she was your wife?”
When Pharaoh found out his error why did he not request a return of his presents to Abram?
My ideas are:
that it would not be consonant with the honour of a King to act in such a way.
that because of the dangers of immorality in Egypt, it was essential that Abram and Soroi left immediately.
(related to Why did Avraham take gifts from Pharaoh, but not the King of Sodom?; see msh210's answer in he name of the Hak'sav V'hakabala )