I may be mistaken so correct me if I'm wrong, but from memory I believe there's a shita in gemara (I think Bava Basra perek yesh nochalin, but can't remember offhand), that if a shechiv mera (someone on his deathbed) writes over his property to his wife and his sons, he has equated the wife to all the sons, so the wife gets 1/2 and the sons split the other half. (The other shita holds the wife is just an apotropos for the property.)
Thus, since you hired the dragon to "devour the rabbi and the congregants", you have equated the rabbi to all the congregants, and the principle of "rubo k'kulo" does not apply since it has only fulfilled exactly half the contract.
Additionally, in Bava Metziah (somewhere towards the end, maybe perek hasocher es haumnin?) there is a rule that if contracted workers back out part way through the job, the ruling depends -- if there are other workers to finish the job, "ein alav elah tarumos," i.e. the employer has no legal recourse (but you would not need to pay for the full job). If there are no other workers, the employer is allowed to deceive them, i.e. claim to pay them more for finishing the job, but not actually give more in the end.
So the ruling here depends on whether there are other workers to finish the job. If so, you must hire others, if not, you can deceive the dragon with a promise of higher wages.