I am an accountant and I have a little confusion aging method specially WHY THE EXISTING CREDIT BALANCE OF ALLOWANCE ACCOUNT should be deducted from the desired amount of the allowance? For example if the aging method results a bad bebt of $5000 and there was existing credit balance of $100 then, the amount adusting is $4900(5000-100) simply WHY? WHY SHOULDN'T ENTIRE 5000 BE RECORDED?

Because $5000 is the ending balance of the Allowance account.
So if the credit side is $100, to put the Allowance balance in balance you have to put $4900 on the debit side.

So the entry will be;

Debt exp. Or uncollectible accounts expense $4900
Allowance uncollectible accounts $4900