Umar rejected the document outright, knowing that it was fabricated because the conquest of Khaybar took place in 6 AH, whereas Sa would ibn Mua'dh died in
5 AH just after the Battle of the Trench, and Mu'awiyah embraced Islam in 8 AH, after the conquest of Mecca.
For example, when the second caliph, Umar ibn al-Khattab decided to expel the Jews from Khaybar, some Jewish dignitaries brought a document to Umar attempting to prove that the Prophet had intended that they stay there by exempting them from the jizya(tax on non-Muslims under the rule of Muslims); the document carried the witness of two companions,
Sa would ibn Mua'dh and Mu'awiyah ibn Abi Sufyan.