Hi @Anonymous
Do you want to remove the value of Budget name and fill Total Budget available with the value that Budget name subtract Amount in list2 when list2 update?
If any misunderstanding, please kindly let me know.
I simply created a Flow, you can refer to as below:
This is my list1

This is my list2

Flow in details:


Expression in Budget name: null
Expression in Total Budget available: sub(items('Apply_to_each')?['Budgetname'],triggerOutputs()?['body/Amount'])

When the Expense Code EA4 be modified as below:

List1 will update as below:

Hope the content above may help you.
Best Regards
If my solution helps, then please consider Accept it as the solution to help the other members find it more quickly.