While リーマン is definitely close enough to invite the interpretation of it being a loanword originated from Lehman, it's actually an abbreviation of サラリーマン (Salary Man), a
Japanese-made English that refers to white-collar workers in general.
Given that white-collar workers do still wear suits most of the time in Japan, it does come full circle and ends with a meaning that sorta resembles Lehman, even though in Japanese it's closer to "businessmen" or "men in suits", with no special banker connotation as the English "Lehman" has.