Graeber's examples of early debt are reciprocal. Otherwise, there would be no need to record them. Sure, it's not as transactional as money is, but it's a transaction nonetheless. I'll admit it's not immediate, but I don't think that's a particularly meaningful distinction. It's not particularly difficult to trust a farmer within your community when they say they'll offer you grain in the future during harvest season. They'd risk ostracization if they break their promise, which is close to a death sentence in that era.