I have two tables - Customer -> Invoices
I have a report based on the customer table.
Consider what the related summary field contains: a summary for the last customer (or the first, depending on where you place the grand total part). So this doesn't work (as you have discovered).
It can be done in Customers, but would require additional fields: you would have to bring over the related summary value per customer into the Customer table via a calculation field, then summarize that field.
The simple solution is to base your report layout on the Invoices table – that is where the numbers already reside an can be processed with simpler means; any related fields (now from the Customer table) are needed for display only.
Thank you - Good thought.