Stop auto payments to a collection agency?

My husband agreed to automatic payment arrangements to Midland Credit Management through automatic bank transfers for a charged-off Chase account. We currently can???t afford to make the payments of nearly $150/month (I was upset that my husband agreed to this high of payment anyway). Is there a way to stop the payments or to lower them? Our credit union says we cannot stop the payments since they are ACH payments and they are only able to stop payments on checks. My husband has called the collection agency, but so far his case worker has not returned his call. I'm not sure if I can call Midland (or Chase) to take care of this because my name is not on the account. Short of closing the credit union account the payments are drafted from, is there anything we can do? Thank you for any advice!