Need a good interior design career advice

My boss has promised me to give me minimum $10,000 US Dollars bonus after the project I am working on is completed. However, my Christmas bonus this year is just half of the amount I got last year, and the reason according to my boss is that the company does not make any money this year, so I wonder how I can be sure my boss will not use the same excuse and cut my bonus to half or not to give me any bonus at all after the project is completed?! It just does not make any sense. Moreover, I have been asking my boss to put whatever she promised in writing but she refused. Can anyone give me a good advice whether I should stay and work on the project till the end and hope my boss will not screw me over or I should just resign as soon as I get another job offer?