Is it permissible to give testimony based on another person's account of witnessing an incident that the witness himself did not attend, and to claim attendance at another incident (the payment of the dowry) that he also did not attend, even though the witness is certain of the narrator's truthfulness?

It is not permissible for you to bear witness with this man concerning what he told you, as long as you did not see or hear what happened. This is because bearing witness is only permissible for what you have learned through sight or hearing. Therefore, no matter how certain you are of the truthfulness of your friend, it is not permissible for you to proceed with bearing witness to something you did not know in the manner considered valid by Islamic law. The proof for this is Allah's saying: "Except for those who testify to the truth, and they know" (Quran 43:86), and His saying: "And do not pursue that of which you have no knowledge. Indeed, the hearing, the sight, and the heart - about all of those [one] will be questioned" (Quran 17:36). Ibn Qudamah said: "Bearing witness is not permissible except for what one knows." It is also narrated from Ibn Abbas that the Prophet, peace and blessings be upon him, was asked about bearing witness, and he said: "Do you see the sun?" He said: "Yes." He said: "Bear witness to something similar to it, or leave it." In the Fiqh Encyclopedia, it states: "Among the conditions for bearing witness is that the matter being testified to must be known to the witness at the time of bearing witness."