Dear Marianne,
Thank you for your inquiry.
To achieve this you would have to authorize in the first place the maximum amount and later on manually complete the payment and charge the final amount (it is however not possible to charge a higher amount than the authorized one).
You have the possibility in wallee to set in your connectors 'Payment Completion' to deferred and later on manually charge (capture) the amount.
Let me know if you need precisions about this.