So for accounts payable you can use Manager. For accounts receivable you can use external billing system.
You can create custom account called
Accounts receivable and make a journal entry once a month to adjust this account for the movement.
For example, if your previous accounts receivable balance in Manager was $10,000 and now it’s $15,000 then you would create journal entry to:
Accounts receivable by $5,000
Sales by $5,000
This will bring accounts receivable in Manager to $15,000 which should be the same as per your external billing system.
If one month later your accounts receivable balance drops to $8,000 then journal entry should be:
Accounts receivable by $7,000
Sales by $7,000
So it can be simple as this.