They want the change done now and won't agree a price first

A change can be properly instructed, in writing, by the right person, and still have no price. You send a quotation, and the reply says get on with it and the value will be agreed later. By the time anyone looks at it, the work is built and your leverage has gone.

Your subcontract sets the rules: whether a quotation comes first, how a change is valued with no agreed price, and what records the valuation needs. Left alone, your main contractor's surveyor values it, often at the nearest rate in your subcontract, whatever the change involved.

Going ahead without an agreed price is often unavoidable. Going ahead without records is what hurts. Without the instruction, your quotation, the working conditions and the resources used on file, that valuation is hard to challenge.

Confirm the instruction in writing and attach your quotation. Say you are going ahead at your main contractor's request, with the price to be agreed under the subcontract's valuation rules. Open a file and a cost code for the change the same day. Then the labour, plant and materials it uses are recorded apart from your base scope from the first shift.

Unpaid Variation Prevention builds that confirmation, file and cost code into your routine for every change. Where the work is done and the price is still open, Variation Claim fixes the valuation rule from your subcontract. It tests the quantities and records, then presses the valuation with a schedule your main contractor has to answer line by line.