There was nothing about the Opium War that was Free Trade.
And no there were no slaves of their own free choice where you come up with that doozey is of interest. Well unless you are speaking of submissive sex slaves.
OK, are you for freedom of contracts? Isn't the freedom of contracts a part of free market?
In ancient Rome (and other places) people were able to borrow money using their own freedom as the collateral. So slavery is a natural and logical consequence of the free market.
And here you have one interesting historical example:
"[...]Before the introduction of coined money the peasant farmer borrowed commodities and repaid the loan in kind, and … was probably able to meet the obligation without great difficulty; but after the introduction of coined money the situation became decidedly more difficult…he must take a loan of money to purchase his necessary supplies at a time when money was cheap and commodities dear. When a year of plenty came and he undertook to repay the loan, commodities were cheap and money was dear", wrote Professor Calhoun.
Unable to get out of debt, eventually bad weather or a poor harvest would bring foreclosure on their land and even bind them into slavery. This enslavement grew to crisis proportions, when Solon came to Athens rescue with his "Seisachtheia" or "shaking off" of burdens. Personal slavery was no longer allowed as security for debts. He canceled such existing debt contracts; and gave back land which had been seized. Farmers who had been sold into slavery abroad by those to whom they owed money were "bought" back and returned to Athens. [...]"
(A Brief History of Interest)