Clincher

n. decisive act or remark (Slang)

Noun
1. an argument that is conclusive
(synonym) determiner, determining factor
(hypernym) argument, statement
2. a point or fact or remark that settles something conclusively
(synonym) decisive factor
(hypernym) determinant, determiner, determinative, determining factor, causal factor
(derivation) clinch
3. a tool used to clinch nails or bolts or rivets
(hypernym) tool

(n.)
That which ends a dispute or controversy; a decisive argument.
  

(n.)
One who, or that which, clinches; that which holds fast.
  

an additional inducement offered to a potential buyer by a salesperson in order to close a sale; inducements might include price discounts, rebates, extended credit, reduced delivery charges, etc.