This would result in erroneous balances being shown for past records. The balance after receipt #1 was not 600. My question was not actually meant to obtain your suggestion about what should be shown. It was meant to cause you to think about the situation so you would realize why this should not be done.
I am not sure I completely understand your last paragraph. If you are referring to having the invoice balance show when choosing where to post a receipt or payment, that has been discussed in another thread that is already in the ideas list: Remove "Paid in full" invoices from the drop down list - #30 by d3mad.