What does the French expression 'avez vous l'appoint' mean?

"Avez-vous l'appoint ?" means "do you have the (smaller) money". This is a question heard when a seller asks you if you could pay the exact amount, without him having to give you anything back.

Avoir l'appoint: to have the exact sum, not an approximative one.

Faire l'appoint: to give the extra cents or small coins that are needed to get to the exact sum.