You didn't specify the currency. Could as well be Russian rubles or even Armenian drams.
The exploit relied partially on the negotiations being conducted in Cantonese, so the matron of honor asked for "500 lumps [of money]", with most/all currency using "lump" as the measure word. The same thing could have been done in English, had he figured out which of the world's various currencies called "dollar" had the lowest exchange rate vs. the USD. The exploit was helped by the fact that Cantonese treats currencies more uniformly than English, so it's syntactically easier to substitute a different currency.
(My friend just went to a local bank and asked which currency they had on hand that had the lowest exchange rate vs. the USD. It happened to be VND.)
While I've got you... I've heard "ya man" for HK$ 20 in the North Point produce market in Hong Kong. One of my friends said that this is a "local count", but didn't give specifics. I'm familiar with the normal "yi sap" for 20. When is "ya" used for 20 in Cantonese? (Side note, 20 is "yi sip" in Thai, despite "yi" not being 2 in Thai... an interesting borrow/carry-over from Southern Chinese dialects.) Is "local count" something more formal than slang? I know it's written as two tens in a single character, but I'm guessing Canto slang also has dedicated characters in some cases.
It was definitely Cantonese. My buddy's father-in-law only speaks Cantonese (grew up in Manhattan's Chinatown, poor enough English that he thought his son-in-law Dave's name was "Day" for long enough to spell it that way on his new-year's red envelope) and his daughters are bilingual in Canto and English.