Supplying Prefer: return=minimal HTTP header will prevent receiving resource item body in the response. This is useful to minimize traffic, when we know that server will not modify the resource item further. However this is not guaranteed, because rest-layer has hooks, that can modify the resource.
My suggestion is, when such modification is detected, instead of returning 204, we return 200 with the full resource item body.
We can have such detection mechanism via check-summing the payload, before and after the hooks execution as @smyrman suggested.