Get Books Outstanding Totals

What customers owe the client ('receivables') and what the client owes suppliers ('payables'), as two totals read from the client's balance sheet as at today. The link's 'externalUrl' opens the organisation in the platform. For platforms whose metadata says 'supportsBooksTransactionDetail' is false: their terms allow only report-level data from a client's books, so the invoices, bills, ageing and contacts endpoints answer 400 there and this stands in for them. Other platforms answer this one with 400. Unlike the ageing, credit notes and unallocated payments are netted off, as on the balance sheet.