According to Tumbaburro de la Picardia Mexicana (Jimenez, A. Editorial Diana, 1977), a book that compiles hundreds of Mexican slang expressions, "cueros de rana" would be...
"...a 100 pesos banknote that had a brownish colour and went out of print in 1975".

Incidentally, and according to Bank of Mexico, from 1925 and 1978 Mexican banknotes were printed by the American Bank Note Company from New York, USA, before giving way to the Banknote Factory of Bank Of Mexico. Perhaps that would explain why some Mexicans nowadays say "cueros de rana" when talking about 'US dollars'? ;)
In any case, I never heard of that expression. I know money can be referred as "varo", "varos", "morlaca", "borrega", "lana", "feria", "billete", "pachocha", "billullos", "morlacos"... I guess it depends the region we are talking about (Mexico is a big country).