After reconversion to Islam, Abdullah b Sa'ad proved to be a good Muslim. He allowed all the injunctions of Islam. He participated in the various battles, and gave a good account of himself. He was intelligent, and was particularly good in revenue collection. Umar was a good judge of men, and impressed by Abdullah's abilities, he appointed him as a Governor of Upper Egypt. As Governor Abdullah b Sa'ad did well and Umar was well pleased with him.
"When a person dies, his works end, except for three: ongoing charity, knowledge that is benefited from, and a righteous child who prays for him."
Prophet Mohammed (PBUH)
"The best of what a man leaves behind are three: a righteous child who supplicates for him, ongoing charity the reward of which reaches him, and knowledge that is acted upon after him."
Sunan Ibn Mājah
"Every day two angels come down from Heaven and one of them says, 'O Allah! Compensate every person who spends in Your Cause,' and the other (angel) says, 'O Allah! Destroy every miser.'"
Sahih Bukhari