“surety-like to write for me, / Under that bond”
A surety (pronounced 'shoor-it-ee') was a person who formally guaranteed someone else's debt: if the original debtor could not pay, the surety was legally obligated to pay instead. The friend signed himself as the poet's guarantor — 'wrote for' him, meaning co-signed the bond — and is now as tightly bound by it as the poet ever was.
historical Usury and the bond- TLN 1870historical topical
“The statute of thy beauty thou wilt take”
A 'statute' (short for 'statute staple') was an Elizabethan legal instrument that gave a creditor immediate rights over a debtor's land and goods upon default, without further court proceedings. The poet says the Dark Lady will invoke the full legal force of her beauty to seize everything owed her. Her beauty is the instrument of debt, and she will enforce it to the last penny.
historical Usury and the bond - TLN 1871historical topical
“Thou usurer, that putt'st forth all to use”
A usurer was a moneylender who charged interest — condemned by the medieval Church and by Aristotle as morally corrupt, because money is 'barren' and should not breed more money. 'Putt'st forth all to use' means lends out everything at interest: the Dark Lady deploys every person and every asset for gain, withholding nothing for free. The word 'use' (= interest, profit from lending) is repeated from the financial conceit that runs through the whole sonnet.
historical Usury and the bond