I think even that is way over the line. Having to show the review means it's likely to drive the reviews up. I'd probably add a star or two because I'm afraid

I think even that is way over the line. Having to show the review means it's likely to drive the reviews up. I'd probably add a star or two because I'm afraid of conflict. And if the reviews give a discount, you encourage first time buyers to give a (made up) review before buying. The only acceptable way to do it is "Please give us a review at [site]". No discounts or promotions.