How to write this expression - sum based on Customer number/amount

[Tiger1]

I need an expression to help me sum customer transactions. In my table i have a list of customers. They have a unique id (Customer Number). The table shows all transactions done that month PER customer. I need a virtual column that sums the transactions per customer.

Then i need to reference another table that shows the same Customers, customer number - their balance. I need to calculate (while referencing between these tables):

Customer Balance / Total of transactions (from the first table i need to total) - PER CUSTOMER