Romans 3.7a is a metaphor: “if through my lie God’s truth abounds to his glory…”  It is a metaphor because the unfaithfulness of Israel (which Paul personifies ahead of Romans 7) was not simply a literal lie.

The reality is found in Romans 11: “For just as you were at one time disobedient to God but now have received mercy because of their disobedience, so they too have now been disobedient in order that by the mercy shown to you they also may now receive mercy.”

Israel’s unfaithfulness was killing Jesus (and covering for it by rejecting the Gospel).  Their unfaithfulness was used to bring salvation to the whole world.