It's easy to see God's promises and His law as competing forces. But Paul shows they work together, not against each other, to bring about God's salvation.
The core tension Paul addresses is whether God's Law and His Promises, given to Abraham and fulfilled in Christ, are in conflict. He forcefully denies this! The law wasn't given to overthrow God's promises, but to prepare the way for them. As Barnes suggests, the law was 'auxiliary to that' plan. It was a necessary step in God's unfolding redemptive story. Meyer highlights that if the law could have procured life, then it would indeed be the source of righteousness. But since it cannot, the promises (which do lead to life and righteousness through faith) remain fully valid and central. The law reveals our inability, driving us to the promises of God, which are freely given and received by faith in Christ.