There is no such thing as perfect crime because no one is perfect. Volpone and Mosca are the ultimate con artists. When they work together they can pull off any trick and can deceive anyone, anytime. But both men have their faults and this is what causes their fall. Neither of them recognizes that they achieved everything together and can continue only if they are co-operating. Volpone doesn’t appreciate Mosca and Mosca thinks that he can go on alone without Volpone.
Volpone is brilliant in deceiving people, and also, he is enjoying every moment of his tricks. But like a gambler (that he is) doesn’t know where to stop. He is doing his cons for the act itself, not because of greed. He does love his treasure very much; in a blasphemous way he even replaces God with it at the beginning of the play. But what he really likes is to deceive people, to play with them, to control them and then take away whatever valuable they have. This is above all the treasures that he already has. In fact he is giving away everything to Mosca in his will just to pull his new prank on the greedy “carrion birds” and the “she-wolf.” He has his ultimate trust in Mosca and he would never expect Mosca (or furthermore, anyone) to deceive him. He admires his servant’s brilliance in pulling tricks but he fails to recognize his ambition to be more than his sidekick. Mosca for him is just a device (just like his disguise costumes) that helps him to perfectly pull his tricks.
On the other hand Mosca wants to be recognized as (at least) equal to Volpone. He wants to be a true partner in their business venture of crimes. He is not as obsessed by the act itself as Volpone is. He knows that he is good as a con artist and values himself even above of his master. We can see that he is a real pro when he is covering up after Volpone’s failure with Celia, but his true brilliance comes when he is finally in charge. While he is making the inventory of all the valuables he shows who the real boss is now. He kicks out all of those who thought that they have any chance to inherit anything. (Volpone enjoys the show so much and he fails to recognize what is really going on.) Mosca ultimately fails in the courtroom because of his over-confidence. He thinks that he is in total control now. He got what he wanted; he showed what he is capable of, and finally his master also recognizes that they are equal; he wants half of everything and Volpone willing to give it. Together they would get away this time too, but now Mosca is not cooperating and this leads to their ultimate fall.
Mosca is not cooperating, as you say, Zsolt, when he tells Volpone that he can’t let him have half the money, and then Volpone doesn’t cooperate by refusing to stoop any lower. You capture the relationship between them very well.
It is as if Mosca wants more credit for being an accomplice to Volpone’s crimes. This is possibly the most corrupt ambition of any of the characters encountered in our readings. The only thing he stands to gain is notoriety.