He had been set free of the bear, and had been taken captive by the man. So quick had been the trade that he had not even made a decision in the matter. From the arms of the beast into the armless debt of Lacey Pollard, he had moved without deciding.
"Armless debt" the intangible obligation Mooney now feels towards Lacey. The contrast between the physical danger of the bear and now, the moral debt Mooney owes Lacey for saving him despite the conflict over their relations with Lorry.