In traditional forms of English dating from the 19th century, man of straw was a figurative expression meaning a businessman who had no financial substance behind him, in other words someone who's appearance - as a merchant or businessman - seemed on the face of it to be sound, but who in reality was impecunious.
He would seem to all outward appearances to be financially robust -- as solid as the Bank of England. But if put to the test, he actually had all the substance and solidity of a scarecrow -- a man literally made of straw. We might today describe him as having all the solidity of a soap bubble.
The expression seems to have been in vogue in the 1830s, at the time of the South Sea Bubble, the greatest financial scandle of the 19th Century, when the South Sea company collapsed, with huge debts, to the financial ruin of many of its investors.