If the contract was done without witnesses, as stated, then it is not permissible for anyone to add one or two witnesses to it, because this is lying and false witness. The presence of a witness’s name on the contract means that he was present when it was done and witnessed the proposal and acceptance. If that did not happen, then affirming it now is false witness and lying. The fact that the witnesses are certain that the transaction took place does not make it permissible to add their testimony to a contract at which they were not present.
The person who has been wronged has to strive to take his rights in the way prescribed in sharee’ah, without lying or fabricating.
What has been narrated concerning giving false witness is no secret. Al-Bukhaari (5976) and Muslim (87) narrated that Abu Bakrah (may Allaah be pleased with him) said: the Messenger of Allaah (blessings and peace of Allaah be upon him) said: “Shall I not tell you of the worst of major sins?” – three times – “Associating others with Allaah, disobeying one’s parents, and bearing false witness – or false speech.” The Messenger of Allaah (blessings and peace of Allaah be upon him) was lying down, then he sat up and kept repeating it until we said: Would that he might fall silent [because they felt compassion for him because of the fervour with which he spoke the words, and they feared that he might exhaust himself].
We ask Allaah to keep us safe and sound.
And Allaah knows best.