The determination of in or out of scope depends on how explicit your supporting documents are, including your requirements baseline, spec sheets, architecture designs, engineering designs, etc., as well as how your contract is written. It is impossible for participants on this exchange to make this determination for you unless you want to upload all of these documents for us to read.
So this becomes about process...the change process specifically. Your team needs to come together and discuss this function B and review your supporting documents and then arrive at a position. Then sit down with your customer / product owner and discuss your case and uncover their position. If everyone agrees it is out of scope, and everyone agrees it is required, then trigger your change process and get it approved, paid for, scheduled, etc. Then carry on.
If they disagree and believes it is in scope, then you have a contract dispute. This triggers your escalation / issue process.