Buy money is self explanatory. This is the money used to purchase something. The only time that I've ever heard buy money was in Scarface. Buy money described the money being used to purchase the cocaine, in heavy quantity. Buy money is usually used to describe the money used in purchasing narcotics.
"... And if you don't come up with the buy money? Ooh, pobre-sito..."
-Someone above Scarface in his first deal, Scarface