In "Heart of Darkness", who says, "I authorize you to take all the risks," and why does he say this?

As the boat approaches Kurtz's station, the men see natives gathering on the banks of the river. Fearing they will attack, the manager tells Marlow that he is "authoriz[ing] him to take all the risks" by making a pre-emptive attack on them. Marlow, however, responds with, "I refuse to take any."