If the pre-authorisation amount is insufficient to cover the total cost of the transaction, you can use the Incremental Authorisation feature.

If the cardholder's actual spending exceeds the initial authorisation, then an incremental authorisation request can be made to increase the authorised amount. 

This method allows for additional funds to be reserved in order to cover the remaining cost of the transaction.
Incremental authorisations are typically based on revised estimates – or higher expenditure expectations – and are used to increase the initial pre-authorised amount.

For more information, see What is Incremental Authorisation?