You could reset the balances to 0 but it would have to be done manually.
You see, the negative balance in this case is correct, i.e. there are 50 DOGE missing from this user’s account due to the bug not being fixed at the time when the user did their withdrawal.
One way would be to set the fees of all deposits before that time to zero. This is how you would do it from your MySQL console, assuming your DB prefix is wp_:
1. Backup your database
2. Assuming that you upgraded to 3.6.4 on the 2nd of September, execute this:
UPDATE wp_wallets_txs SET fee = 0 WHERE category='deposit' AND created_time <= '2018-09-02';
This would force all of the deposits before that date to not have any fees.
Hope this helps. Again, my apologies for the mistake on my part.
Let me know if you have any further questions about this.
kind regards