If this happens your first responsibility is to inform your team and to clearly explain the cost of the decrease in quality. After all, your understanding of it should be much better than your boss's understanding. Make it clear what is being lost and what is being gained, and at what cost the lost ground will be regained in the next cycle. In this, the visibility provided by a good project plan should be helpful. If the quality tradeoff affects the quality assurance effort, point that out (both to your boss and quality assurance people). If the quality tradeoff will lead to more bugs being reported after the quality assurance period, point that out.