It might be legal but it's against Google's policies I'm afraid:
> Contributions to Google Maps should reflect a genuine experience at a place or business. Fake engagement is not allowed and will be removed. This includes: [...] Content that has been incentivised by a business in exchange for discounts, free-of-charge goods and/or services.
> Contributions to Google Maps should reflect a genuine experience at a place or business. Fake engagement is not allowed and will be removed. This includes: [...] Content that has been incentivised by a business in exchange for discounts, free-of-charge goods and/or services.
https://support.google.com/local-guides/answer/7400114