disable mandatory field?

disable mandatory field?

I want to conditionally disable a mandatory field, but can't get this to work!  I have three payment options.  If someone selects "Credit Card", I want the PO# and Account# fields do disable, but this doesn't seem to be working.  I currently have PO# set to be a require field, and this seems to be causing the problem.   

Maybe I should simply define the goal instead of the problem:

I want the user to pick either Credit Card, PO or Acct# - this is a required selection.  I then want a new field to appear which requires the user to input the selected number (either Acct or PO, for example).  

I failed to make this work by using the 'hide' and 'disable' functions conditional on the pull-down payment option.  Any assistance is appreciated.