Google doesn't make laws, so there's no legal question. The question is whether it's against Google's policy, which is an entirely different situation.
Basically what you have is 2 versions of your app, one that's free and ad-supported, the other one is not free and has no ads. If that were against Google policy, there would be thousands of apps pulled from Play. Don't issue a "coupon", use an in-app purchase that converts the app to the ad-free version, or put the 2 versions in Play, the way many other apps do. Then people are free to install the free version or pay for the add-free version. (And make the paid version just update the add-supported version.)