Was OT wrong on this question?

Dear littlestart

The question said that balance of the receivables account includes a cash received of 1,800 which had previously been written off, it means that the received cash incorrectly credited in the account while it doesn’t have any effects on this account, therefor it should be added to the receivables balance.
Additionally, in respect of your second doubtful point, when a business receives a debt which it was an allowed debt, the entry of this receiving just affect on credit side of receivables account and on debit side of the cash or bank accounts too, so as the question said that the receivables account includes of this entry it means this event doesn’t have any revisory effect on the account’s balance because it was accurately and truly deducted before.

Good luck