Document Change Rules for Line Items will help you to control which field will be editable after posting. Examples of such fields could be due date, payment block, payment method, text fields, etc.
IMG Path: Financial Accounting (New) -> Financial Accounting Global Settings (New) -> Document -> Rules for Changing Documents -> Document Change Rules, Line Item
When you enter into the transaction you will have the option to either create a new rule or to edit an already existing one.
Maintain the necessary fields as required.
After saving you will have additional options related to the section – “Stipulation for changing”.