Beppi is correct, the offered salary is most likely much more important.
However, whenever MOM asks about income, they are talking about your fixed income before bonuses, commissions, etc. (Yes, this can screw over sales people.)
When MOM asked for my income for my EP (actually, an agency asked who filled out my application forms), I don't believe I had to provide any documents to demonstrate said income. That said, it is a crime to lie. If MOM catches you, you will lose your pass, be deported, most likely barred from future entry, and possibly face prison time.