Most agencies do not allow their employees to purchase any abandoned property due to conflict of interest. My old agency, we had an officer who was buying vehicles from the contract tow company after they were abandoned and he got into trouble for that as the agency felt there was conflict with officers towing vehicles and then obtaining them from the tow company. I agreed with that decision.
Most times in that situation you described, I would put the knife in the trunk and the person could retrieve it after they or I left. I did the same with mace and other such chemical sprays and I've seen other officers do similar in similar circumstances.