How to settle the issue of quoted prices once the contract is signed? How will the adjustment be treated in books?

If there is no price adjustment provision in the contract, which seems to be the case from the logic of the question, then you the contract will have to be implemented with no adjustment. However, in the extreme cases the issue is dealt with in the civil law of the country of the applicable law provided the law provisions for justification of the case are satisfied. 

In the other case price adjustment provisions should be applied provided that the conditions for adjustment are satisfied.

Agreed with colleagues answers

But sir in reality they add other clauses and make amendment in the contract. And one amended signed and agreed the change effect the price. In this particular case amendents were made and once the contract reach maturity. On extension it automatically become part of new contract.