Wei Jin and Old Zhou's unexpected visit had already surprised Yang Fan greatly, but when Wei Jin said those words, Yang Fan was momentarily stunned, his first reaction being that this favor seemed a bit too big to repay.
Since Wei Jin went out of his way to seek out Old Zhou and have him find this place to discuss the matter in person, Yang Fan could be certain that the matter was nearly settled.
People with little real power tend to make empty promises, treating things that may not, and sometimes definitely will not, be accomplished as if they certainly will.
However, for someone like Wei Jin, even if things are almost a sure bet, he might still leave some room for flexibility. Therefore, when he offers such a definitive answer, no matter how mysterious the process has been, it's a done deal.
Wei Jin really gave Yang Fan full face with this.