Business pages offer new tools like hangouts and hangouts on air to connect and interact with your customers. In order to ensure quality of the network and compliance with all applicable federal, state and local laws, google+ has set some promotion policies.
One of the important policy to note is you are not allowed to directly run any contests, sweepstakes, offers, coupons or promotions on your google+ page.
However they allow to share links to pages that may contain any of the above (they reserve the right to remove posts / content that has direct promotional material). Just want to give a heads up before you scratch your head why that post disappeared.
Here is the google+ pages contest and promotion policy
Here is the best practice : If you decide to run a contest, sweepstakes, coupon or promotion just create details about your offerings on your own website, blog or pages outside google+ and share the link on google+ pages.
The result is same but you are playing safe by adhering to promotion policies. Hope this helps while planning your next promotion campaign.