The Gemora (Kiddushin 41b) cites a Mishna: If one sends out a fire in the hands of a deaf-mute, an imbecile or a minor (and it consequently burned someone’s haystack), he is not liable to pay according to the laws of man, but he is liable according to the laws of Heaven. If, however, he sent out the fire in the hands of a competent person, the competent person is liable to pay for the damages.
It would seem that in the case where the sender sent the fire with a competent person, the sender is not liable at all, even under the laws of Heaven!
The Ram”a (C”M: 32:2) rules that if one sends out false witnesses to testify against someone, and they cause that fellow a loss, the sender is not liable at all, even under the laws of Heaven. This is because we say that there cannot be a shliach to commit a transgression.
The Sha”ch disagrees and maintains that the sender will be liable to pay under the laws of Heaven. He explains the distinction between the two cases. The sender will always be liable under the laws of Heaven. The only reason that the sender is not required to pay at all in the case of the fire is because once the competent person is liable to pay, there is no place for the sender to be liable as well!