Separate the cost claim from the cost. Cost claim is what has been notified to the subcontractor, while Cost is what goes to the final forecast. Leave the cost at 0 until you agree.
How to do it
Go to Change Order, select the Counter Claim tab and register the counter claim with the cost claim you have notified in the Cost claim column.
Leave the Cost column on the lines at 0.
When you agree, set the status to Approved on the lines, or use Set all. The cost is then set equal to the claim.
If you agreed on a different amount, write the cost down to the agreed amount and save.
Good to know
The counter claim is registered and visible even though it doesn't affect the forecast yet.
You can always adjust the cost afterwards.
A counter claim can also be carried forward as a change order against another subcontractor, for example when another subcontractor has to rectify the work.