validation limit values

Hi,

Is there a way to set the validate_numericality_of limit to a variable
passed in from the controller? I have this model:

class Vision < ActiveRecord::Base

  belongs_to :employee
  belongs_to :users

  validates_numericality_of :paid_amount, :on
=> :update, :less_than_or_equal_to => 250, :allow_nil => true
  validates_numericality_of :cost, :allow_nil => false

end

I'd like to use something like: :less_than_or_equal_to=> :variable

Thanks.