before adjusting and closing the accounts had occurred, the Allowance for doubtful showed a debit balance of $3,200. Aging of the accounts receivable probably uncollectible to be $2,100. Under these circumstances, year end adjusting entry for uncollectible accounts expense would be?

The difference of $1,100 should be made to fall free.
This means you reverse the said amount to
bring the situation closer to reality.