Green’s Dictionary of Slang

1. the purchase of a drug.

1930		J. Tully Shadows of Men 213: When too destitute to ‘make a buy’ [...] he would seek other addicts and tell them marvelous tales until they would give him a ‘jolt’.
1934		Current Hist. 181–4: A jury is usually loath to convict a Professional man, and if only one or more ‘buys’ have been made by the agent [...] he is usually acquitted.
1953		‘William Lee’ Junkie (1966) 69: The new pigeon is given marked money and sent out to make a buy.
1967		M. Braly On the Yard (2002) 239: He had been chippying with heavy even before chilly contacted him, and when his money grew long he was able to make several large buys.
1974		V.E. Smith Jones Men 57: Helped us get a buy at that Harper Street Place.
1981		J. Ellroy Brown’s Requiem 77: We could have pulled it off low-key [...] and making a discreet buy.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 34: You make the buy.
2013		D.A. Summers ‘Nothing to Lose’ in ThugLit Nov.-Dec. [ebook] ‘[A] couple of brothers have a lab out there [...] Whenever Rico starts to run low [...] he just drives out there and makes a buy’.

2. money required to purchase a quantity of drugs.

1993

R. Shell Iced 71: He had to sell the rest to get his ‘buy’ (money to buy a quanitity of rock) money.