Pricing should not be changed in billing document?

HI,

We have given manual entry provision for pricing in Sales order level . But as per the company requirement, the price should not be changed in the billing document. Also the point to be considered is, it is a delivery based billing. What is the configuration required to ensure that , System should not allow pricing to be changed in billing document when it is getting created.

I have checked the forum and found no threads linked to it. Kindly help me in this regard.

Thanks in advance

Rgds,

Sivarajesh. J