He will give the Devil his due
Literally, pay the devil what you owe him. Used figuratively to mean 'give back what you owe', either money or favours.
From Shakespeare's Henry IV Part 1, 1597:
Constable: I will cap that proverb with 'There is flattery in friendship.'
Orleans: And I will take up that with 'Give the devil his due.'
See other phrases and sayings from Shakespeare.