Based on the Google policy beeneeb quoted, it is a clear violation to give away items (i..e pay) for a POSITIVE review.

If you were to approach your customers and offer a giveaway item for simply completing a review, without any suggestion that the review is positive, then I don't see any Google violation.

This approach also has the benefit that most of your site's clients will infer that the gift is for a positive review, and offer one.

The drawbacks are you could give away items for a bad review. The other risk is a trigger-happy Google employee could take action against your site. If that happens, you would have to clearly explain that you did not violate the policy and request any punishment to be lifted.