adjust a written off gift

Howdy - we've had a spot of trouble with chargebacks from credit card companies.  The gist of it is:

- donor agrees to donate $15/mo for 12 mo for a total pledge of $180.  Six months later, donor says they didn't agree to this and credit card company gives them back their money for 3 months.  Pledge is adjusted down to $45 with the rest written off.  Now, credit card company has given donor back one more payment of $15 so we need to take is gift down to $30.

But since the pledge balance has been written off, we cannot adjust the pledge to remove $15 from his payments and knock his pledge down to $30.  Is there a way to adjust a written off pledge?

Thanks all!