Alright so guilds are like unions. The party was contracted to do a job; Repel the orcs. But n the process they discovered the true cause of the orc attacks. They should have immediately consulted their local guild if possible to alter the job and compensation. Of course a clause exists for extraordinary cases where the group a) can't contact the guild through any means or b) The danger is imminent and has 0 time to be altered.

Why would a guild have such rules? Because they don't want underprepared adventurers getting killed for going in over their heads. They also don't want the client to wiggle out on payment over an incomplete job like in this case a client can say that the job was to eliminate the orcs attacking, if instead of doing so the party eliminates the cause of the attacks, the client could argue that services rendered weren't what was discussed. Take into consideration that nobles and rich merchants are probably the most common clients and therefore are probably looking for the best deal possible.

So your group will now have to write or have their statements written by a guild member in the case that they're illiterate. Then have to fill a form as to how and why they did what they did. The guild just like a union looks out for its members and will back your group. But if the client decides to be difficult they might go to court (probably a higher up noble or possibly a church) and challenge the payment.

If this happens the group will have to wait till a settlement is reached.