You'd have to take gold or silver because your paper money or modern coinage would be worthless, I doubt anybody would except a $20 bill that has an unknown design or future date.
your right. so buying gold in the past isn't going to work. Someone would have to buy a coin whose value is in it's rarity not intrinsic value or take back in time another commodity that would have been scarce and expensive back then.
I suppose you could bring plain gold bars back, have them assayed, then get high-grade coins of choice. I want the Victoria "Una and the Lion" 5 pound gold coin from 1839.
I'd let my passenger pick the when and I'd bring: 1970's = $5,000 silver cert's and buy the Virginia Hoard of Confed Currency 1935 to 1964 = Spend 10,000 1935 silver cert's on nice gold for the money 1800's = almanac for some betting to buy "worthless land" 1770's = Colonial Currency for Nice Large Cents and Chain's