Is what the questioner did, giving testimony and swearing an oath in court regarding an event he did not witness, with the intention of preserving a right, correct or incorrect, and what is the expiation for the oath if it was incorrect?

The matter of bearing witness is grave, and a Muslim must strive for truthfulness and accuracy in it, testifying only to what he is certain is true. If sufficient knowledge and complete certainty are attained through the convergence of reports and circumstantial evidence that the subject of testimony is the truth, and direct observation is not required, then the testimony is valid, for testimony is linked to knowledge both rationally and legally. However, if complete knowledge is not attained, or if the testimony contains something that contradicts the truth, then it is false witness, which is a major sin necessitating sincere repentance, and its perpetrator is liable for restitution and punishment.