Seisachtheia

Seisachtheia (, from σείειν "seiein", to shake, and ἄχθος "achthos", burden, i.e. the relief of burdens) was a set of laws instituted by the Athenian lawmaker Solon (c. 638 BC–558 BC) in order to rectify the widespread serfdom and slavery that had run rampant in Athens by the 6th century BC, by debt relief.
Debt in Athenian society.
Under the pre-existing legal status, according to the account of the "Constitution of the Athenians" attributed to Aristotle, debtors unable to repay their creditors would surrender their land to them, then becoming "hektemoroi", i.e. serfs who cultivated what used to be their own land and gave one sixth of produce to their creditors. 
Should the debt exceed the perceived value of debtor's total assets, then the debtor and his family would become the creditor's slaves as well. The same would result if a man defaulted on a debt whose collateral was the debtor's personal freedom.
Seisachtheia reforms.
The "seisachtheia" laws immediately cancelled all outstanding debts, retroactively emancipated all previously enslaved debtors, reinstated all confiscated serf property to the "hektemoroi", and forbade the use of personal freedom as collateral in all future debts. The laws instituted a ceiling to maximum property size – regardless of the legality of its acquisition (i.e. by marriage), meant to prevent excessive accumulation of land by powerful families.