don’t count your chickens before the eggs have hatched

Do not plan something as the event may not occur.

Don’t count your chickens before the eggs have hatched as the deal may not happen.

A: "I'm going to go to Hawaii when my bonus comes in." B: "Don’t count your chickens before the eggs have hatched."

A: "Here's the list of the things I'm going to buy next month when I get money." B: "It looks great, but don’t count your chickens before the eggs have hatched."