ACT III SCENE 3: Why won’t Shylock accept six thousand ducats?

BASSANIO: (to SHYLOCK) For thy three thousand ducats here is six. SHYLOCK: If every ducat in six thousand ducats Were in six parts, and every part a ducat, I would not draw them. I would have my bond. DUKE: Upon my power I may dismiss this court, Unless Bellario, a learnèd doctor, Whom I have sent for to determine this, Come here today.