General Motors Co’s (GM.N) decision this week to give back up to $10 billion to shareholders over the next two years will likely delay one of its important goals: achieving a top-tier credit rating that would benefit its growing auto finance unit. The No. 1 U.S. automaker has been building its GM Financial unit through acquisitions and more aggressive marketing to dealers. Reaching a single A credit rating would give GM greater flexibility to fund GM Financial at lower cost.