When the total of the invoices linked to a contract exceeds the contract amount (including any planned changes), posted exceeds the Final Forecast for the contract. You then need to clarify what the excess amount actually is.
Three ways to solve it
Increase the contract amount. If the excess is a genuine additional cost on the contract, increase the contract. To stay in control, you can add the excess amount as a new cost line on the contract instead of changing the original line.
Split an invoice. If parts of an invoice belong to another contract or change, split the invoice and choose the correct link and amount for each part.
Register a change. If some of the invoices belong to a change that is not registered on the contract, create a new Change Order Sub and link the invoices to it. The amount in Cost on the change counts in the final forecast straight away, regardless of status.
Read more about linking and splitting invoices in Invoice linking, and about contracts in Purchasing.
Note: a transfer between production codes does not solve this, because it changes the revised budget and not the Final Forecast. See Why doesn't the Final Forecast change when I transfer amounts between production codes?