I can offer a workaround with Zapier and Google Sheets.
You create a project “customer contract”
You create a Zap - new task created > add to Google Sheets.
This way you can add the numbers in the Sheet and track it there.
I do not recommend this, as it creates another manual step.
Or you can think of using a plugin like Everhour to track time. It integrates with Asana, you can have time budget by Customer, Project etc. It works nice once you set it up.