You should not assume that something will happen until it actually happens. It can be used for a variety of situations: spending money you cannot afford to spend because you expect to receive some money is one example.
別過早打如意算盤。
原文字面義：不要在蛋未孵化前，就先數算［認定］會有幾隻小雞。很多時候，「計畫」趕不上「變化」。那些借錢玩股票的朋友，可要特別小心啊！