A manciple is someone who's in charge of purchasing food and supplies for an institution like a school, monastery or law court. This particular manciple works for an inn of court (the "temple"), which is a place where lawyers might live or gather. Although the Manciple's portrait starts out by holding up this manciple as one who can serve as a great example for other purchasers, we quickly begin to question the guy's ethics when we learn that, whether he purchases with cash or on credit, he always ends up ahead in his accounts. Now how, exactly, is that possible, unless the Manciple is fudging the numbers somewhere? So when the portrait wonders how amazing it is that the unlearned Manciple always comes out financially ahead of his learned masters, it's likely with a bit of irony.
The Host raises more suspicions of the Manciple's dishonesty near the end of the Tales when he warns the Manciple that the Cook may pay back his insults by finding fault with the Manciple's "rekenynges," or financial accounts (Manciple's Prologue 74). The Manciple handles this by giving more wine to an already dangerously-drunken Cook, further throwing his ethics into question.
The Manciple's Portrait is not the only example we get of someone who tricks those who are above him on the totem pole (see the Reeve for another one). His successful face-off against "an heep of lerned men" (General Prologue 577) suggests that there's scholarly savvy, and then there's financial savvy, and sometimes the twain ne'er shall meet. It also raises the question of the types of learning proper to each social class. Is money-handling simply too low-brow for the likely higher-class lawyers to do well? And, moreover, is an unethical nature necessary to financial success and therefore out of reach for noble types like the Knight, who are supposed to be impeccably moral?