The 6th-Case-affix is optionally used in denoting the agent, (but not the object), when the word is governed by a Future Passive Participle (kṛtya).,
By 2.3.65, verbal nouns, (krit-formed), always govern the Genitive in denoting agent or object. The present sutra declares an option as to agent only, in the case of those verbal nouns which are Future Passive Participles. Thus भवतः or भवता कटः कर्तव्यः 3.1.95.
Why do we say 'in denoting the agent'? In denoting the object, no option is allowed; the Genitive is compulsory. As गेयो माणवकः साम्नाम् ॥
Vart:- The prohibition of the Genitive should be stated in the case of the Future Passive Participles of those verbs which govern two objects. Thus क्रष्टव्या ग्रामं शाखा देवदत्तेन, नेतव्या ग्राममजा देवदत्तेन ॥,