Pound of flesh
Something which is owed that is ruthlessly required to be paid back.
This of course derives from Shakespeare's Merchant of Venice, 1596. The insistence by Shylock of the payment of Antonio's flesh is the central plot device of the play:
The pound of flesh which I demand of him Is deerely bought, 'tis mine, and I will haue it.
The figurative use of the phrase to refer to any lawful but nevertheless unreasonable recompense dates to the late 18th century.
See other phrases and sayings from Shakespeare.