Difference between list of balance and ledger control account

I had this question following:

At 1 April 20X9, the payables ledger control account showed a balance of $142,320
At the end of April, the following totals are extracted from the subsidiary book for April
Purchase day book 183,800
Returns outwards day book 27,490
Return inwards day book 13,240
Payments to payables after deducting $1,430 cash discount 196,360

It is also discovered that:
(a) The purchase day book figure is net of sales tax at 17.5%, the other figures all include sales tax
(b) A customer’s balance of 2420 has been offset against his balance of 3650 in the payables ledger
(c) A supplier’s account in payables ledger, with a debit balance of 800 has been include the list of payables as credit balance
What is the corrected balance on the payables ledger control a/c?

I wondered that why the amount of 800 in payables of list of payable balance didn’t entered in the payables ledger account?
Could you explain that and give me the fastest way to settle other question like this?