Is it permissible for two witnesses to sign a contract for the sale of land, when they did not witness the original sale, in order to validate it in court, given that one party (the first party) denies the sale and the other party (the second party) paid a large down payment?

It is not permissible to add witnesses to a contract that was concluded without them, as this would be considered a lie and a falsehood. The presence of a witness's name in a contract implies their attendance and their witnessing of the offer and acceptance. If this did not occur, then asserting it would be a falsehood. The aggrieved party should seek to reclaim their right through legitimate means that do not involve fabrication. The Prophet, peace and blessings be upon him, considered false testimony to be among the gravest of sins.