Hi Witold,
What about making use of summary projects/subprojects?
As long as you dont know whether a customer can be invoiced, all transactions are recorded on a "neutral" project with the type internal or better of a nonchargeable T&M project.
At the time you know the customer you create a new T&m subproject and start invoicing the customer for the transactions that you moved to the subproject via the adjustment function.
Wouldn't that be an option as the project number does not change in this case. You just get a suffix behind.
Best regards,
Ludwig