Have you already contacted the dealership before turning to a lawyer?
Look, tampering with the odometer is straight-up fraud, so you've got plenty of leverage here. With your receipt and invoice in hand, you can start right away with a formal request to the dealership: a registered letter asking for either the sale to be cancelled and full refund, or a price reduction proportional to the difference in mileage (used cars lose value with miles, so 70,000 extra km matters). They might try to claim they didn't know about the tampering, but it's their responsibility to check before selling, not yours.
If the dealership won't budge, take it to a law firm that specializes in automotive disputes: many offer free initial consultations. You're entitled to coverage under the warranty for hidden defects (and an altered instrument is a serious defect) for two years from the sale. The law is on your side, especially with purchase documents that prove the price you paid was based on that mileage claim. If needed, get an official inspection from a trusted mechanic to document the car's actual condition.
Move fast though: the more time passes, the weaker your fraud claim becomes.