Midland Credit Management,Inc. MCM ,Midland Funding LLC convinced me into letting them take payment out of debit card I'm on SSI Los Angeles, California

Was sick that particular morning I am catagorized as a cardiac cripple, had taken my meds not long before the call. couldn't understand very well he had an accent. I never give out my debit account number and will never understand why I did this time. Recieved a letter today stating they will be taking out $50 every 15th of each month for 10 months I dont know what Im going to do I have a very limited income and that SSI is my only income. Will try to stop payment but since it was debit card it may not be possible , would love some advice