Is it true opening balances of advance payments fed from old system are not fetched by ERPnext Sales Invoices. What are the solutions available?
Opening Balances are meant to establish Balance Sheet items as of a Date you like where receivables, payables and inventory can be configured in detail fashion.
You are suppose to be migrating only open sales invoices as receivable against which payments will be expected later on. Previously recorded payments are historic records will be travelling as net balances in different asset accounts within your opening balance sheet.
Please help yourself with the article as following;
Its not a bad idea to raise this as a feature request on Issues · frappe/erpnext · GitHub
You can also include advances paid to suppliers for ERPNext Purchase Invoices
You would need to convert the opening balance to a payment entry. Each customer would need their own entry, so it can be connected to the customer’s balance.
You may consider creating a temporary “cash” account to limit the activity in an account tied to a bank account where the added entries would “muddy-up” the bank reconciliation statement.