There is no one table that would show you a daily AR balance. There are two sets of tables that would show you the AR balance currently. If you wanted a daily AR balance looking back in time, you would have to calculate that.
Current AR balance...
In detail, the transactions are stored in the RM20101, RM Open
In summary, the current balance per customer is stored in the RM00103, Customer Master Summary
Calculating the balance back in time would get pretty complicated with the apply tables, as you would need to essentially back out any posted payments and (if you wanted to be detailed about it) how they were applied. You would still look at the RM20101 and the RM30101, RM History-- but you might also need the RM20201, RM Open Apply (to see how payments were matched to invoices) and the RM30201, RM Apply History. Of course, this is assuming you would just be looking at posted activity.
Take care,
Christina