The answer to this question comes in Chapter 22, entitled "The Holy Fountain." In this chapter the Abbot begs Hank to help fix the fountain, but when he hears that Merlin is already working on this problem, Hank refuses to help because, as he says:
"It will not answer to mix methods, Father; neither would it be professional courtesy. Two of a trade must not underbid each other. We might as well cut rates and be done with it; it would arrive at that in the end. Merlin has the contract; no other magician can touch it till he throws it up."
Thus, because Merlin already has "the contract" to repair the fountain, Hank is happy to let him do the job as part of his "professional courtesy." Note here how we have yet another exmaple of Hank's 20th Century American views being imposed on Arthurian England. The Abbot is at first disconcerted by Hank's refusal to help until he realises that he can "persuade" Merlin to stop working on the fountain and thus Hank can be engaged to work on it by himself, much to the chagrin of Hank.