civil suit recovery of money for extra work

I'm a defendant in a civil suit (recovery of money) in Coimbatore district court. Plaintiff is a building engineer/contractor, who claims money for the extra work and also non performing as per agreement. As per agreement receipts available............... The defendant side given proof for nonperforming work and marked as documents. The court engineer also stated the deduction of pending work as per agreement. For extra work, conditions mentioned in agreement that defendant should give amount to the plaintiff for extra work at that time of work itself. As per plaintiff oral wish, no receipt or additional agreement are entered in between parties. Defendants approached plaintiff to put additional agreement for extra work but the plaintiff refused to put additional work due to income tax problem. Extra work is parallel work at the time of construction work and judge asking documents to prove the amount given by defendant for extra work.