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1.
when you have a formidable proposition and want to close the deal
Don: Lookie here boy, I'll give you $50 for it.

Bill: Done. Cash on the barrel.
by T. Polkinghorne April 06, 2009
 
2.
also "cash on the barrelhead". Means that you give me cash, and I'll give you the product at that time. No credit no delivery.
Buddy: You ever think about selling that old juicer?
Barbara: I'll give it to you for $20 - cash on the barrel.
Buddy: OK, here is the money.
Barbara: Thank you, here is the juicer.
Buddy: It has been a pleasure doing business with you.
Barbara: The feeling is mutual.
by smoo-smoo-smoo October 06, 2010