Note 5 at Gal. 5:15: Every time that anyone advocates Christian liberty, the critics always bring up the same question, "Why then live holy? If God accepts us by grace, then what motivation is there to have godly actions?" Paul addressed this very issue in much detail in Romans 6 (see notes 1-2 at Rom. 6:1-2, pp. 775-776; see notes 7-8 at Rom. 6:15-16, pp. 781-782).

Here, once again, Paul is giving reasons for living a holy life. Even though we do not have to fear God's wrath or rejection, if we walk in strife with others we will reap the negative results that those actions produce. In other words, there are still consequences to our actions outside of God's direct punishment. Mankind and the devil will make us pay dearly for ungodliness (see note 8 at Rom. 6:16, p. 782).

So the liberty that Paul is promoting is freedom from the guilt and condemnation of sin, not freedom to sin. Anyone who chooses to use this liberty in Christ to indulge his flesh will suffer for it. However, it's not God punishing him, it's simply that person reaping what he has sown (Lk. 6:38; Gal. 6:7).