count one's chickens before they're hatched

[count one's chickens before they're hatched] {v. phr.}, {informal} To depend on getting a profit or gain before you have it; make plans that suppose something will happen; be too sure that something will happen. Usually used in negative sentences.

When Jim said that he would be made captain of the team, John told him not to count his chickens before they were hatched.

Maybe some of your customers won't pay, and then where will you be? Don't count your chickens before they're hatched.