Oddly enough, the issue of payment for this task was never raised.
Lol, true. But it's also true they really didn't finish the job... it's impossible to completely clean that basement - there will always be one alive.
Now serious,we start with 500 Meseta, so part of it (or the whole) could be a payment in advance. That would explain why the principal don't let us go outside Piata until we finish the job.