Just getting back from the holidays and picking up on this thread.
To Priya's point, Google doesn't have a problem with soliciting reviews, just incentivizing them.
Each site has such different rules, and Google does seem to change up their support language frequently.
Review sites like G2 Crowd facilitate incentivizing reviews, but they label incentivized reviews on their site so they don't break FTC's rules of transparency.
If each of the incentivized clients/customers were to explicitly state in their review that they were offered an incentive, then it would not be against the FTC regulations, but the catch 22 is that might also draw attention from sites like Yelp and Google that you are incentivizing the reviews which goes against their TOS.
Ultimately, the safest thing (and we recommend) is to combine the offer as a 'thank you' regardless of whether a review is even written or not. Anything else does right the ire of the FTC, Google, Yelp, or other 3rd parties.