Shalt be blessed - Blessed in the “act” of doing good, which furnishes more “happiness” than riches can give, and blessed or rewarded “by God” in the day of judgment.
They cannot recompense thee - They cannot invite you again, and thus pay you; and by inviting “them” you show that you have a “disposition” to do good.
The resurrection of the just - When the just or holy shall be raised from the dead. Then “God” shall reward those who have done good to the poor and needy from love to the Lord Jesus Christ, Matthew 10:42; Matthew 25:34-36.