Yes you can have a “negative” compensation event - assuming this was originally a clear requirement in their Scope. You then assess this as a (negative) compensation event, for which you calculate the forecast cost of not doing this work and then adjust the Prices accordingly. The activity schedule should be amended to reflect the revised total of the Prices. When you say “agree” - as Project Manager you are able to assess yourself (in accordance with the contract) if you do not agree with their quotation.