Hello;

Assuming that the original AR amounts for the customers were not written off from AR and that amounts received were entered as collections on amounts receivable. Then all you are adjusting is the Allowance and the Bad debt expense amounts not touching the main AR account(s). Having a negative expense isn't so much against GAAP, its more that it just doesn't look nice. 

In this case instead of having the allowance account adjustment be recorded against Bad Debt Expense, just record it to the most applicable Other Income account you have.

DR   Allowance account        $30k

CR     Other Income                          $30k

Generally speaking, everyone expects expenses to have debit balances and positive amounts. Though sometimes for internal reporting it makes more sense to leave it as a negative expense as to readily see and understand changes between reporting periods, and then cleaning things separately (in excel or by external year end accountants) up for external financial reporting.