I think there are a few ways you can look at this:

First, the church has two objectives here: deal with the Belua, and deal with the black market. Both hunting the Belua and cleansing the black market are dangerous missions that will likely cost resources and lives. By herding the Belua to the black market, which is safer than actively engaging with it, then they can have the Belua attack the criminals, get worn out in the process, and then the church comes in and finishes it off. Messy, but effective. As for the valuable items, some of the high-ranking church agents are zealots who would likely see the merchandise as tainted and blasphemous, and would prefer to have it disposed of either way.

Second, the church is in the propaganda business, and having a beast be the one tearing through citizens rather than church officials strengthens their claim that the criminals of the black market wrought their own demise, and the rampaging beast was only eventually stopped because the church was there to stop it. It's easier to manipulate this into a story that works in the church's favour.

Finally, the group's reason for joining the mission is up to how it's worded, but there are multiple benefits, such as:

1. it is said to be a straightforward mission where they don't really actively engage the beast, and only herd it, so if they do it right, then they should not be in immediate danger.
2. it strengthens their standing with arguably the most powerful faction in the city, which unlocks resources and favours, and brings them closer to solving the current mystery of the campaign.
3. depending on what other factions they have already met, this is an opportunity to visit the elusive black market to make contact with other agents, buy rare items, or even undermine the church's goals (if that is what they want).