NEC4 Option E - uplifted costs

Good Afternoon,

I have a situation whereby we’re operation under NEC4 Option E contract and during our validation process during month end we found records whereby the Main Contractor has asked his sub-contractors to inflate their previous prices by an additional 30% to account for CIS tax deduction.

Are we obliged to agree to this or can we disallowed this under 11.2 (26) or any other clause ?