Check in Go collections

I defaulted on a Check n Go storefront payday loan in Oklahoma. It was sent to their collections department, and initially they offered me a settlement for less than the balance, with three monthly payments. I made the first payment in May with the prepaid debit card that I had given the Check n Go rep, but missed the June and July payments because I was in the hospital. I called the same guy when I got home and he told me since I missed payments, the settlement was void and I know owed the original amount, but that if I could pay $50 by the end of July he'd set up payments for August and September, and the account wouldn't be sent to another collection agency. I agreed to that, but the next day I noticed some unfamiliar withdrawals from my checking account on June 28 and July 19. The bank told me that Check n Go had initiated an electronic check on those dates. The rep I spoke with the day before never mentioned those payments; in fact, the balance he said I owed did not include one of those payments! So I called Check n Go back, that time they wouldn't let me talk to the same guy but I was transferred around to several people. Finally a guy told me that the person I had spoken with the previous day had made a mistake in setting up another arrangement with me, and that if I didn't pay $50 by the end of business that same day, they'd keep doing the electronic/ACH withdrawals until the balance was paid. I told him I didn't have $50 that day, so could he at least tell me the date and amount of the next ACH withdrawal, because the other two were different amounts. He said he had no idea when or how much, because they have another company that handles it for them.

I know Check n Go is legal in my state, but can they really just keep withdrawing money after we made an agreement to use a debit card on certain dates? Can I still revoke ACH access at this point, or do I have any options? I intended on paying this according to the arrangement I made, but if they try to withdraw money without me knowing the date or amount, my account will probably be overdrawn and I'll end up paying more fees, which I'd like to avoid. I thought once the account went to the collections department, they would work with me, is that not the case with Check n Go?