Anything that weighs less than 20kg was put in the barrel. A feather, a pencil... could have been anything.
'he put something in it and it weighs less than 20kg' <---this sentence could refer to 'it' as the barrel OR the item being placed within it.
Unless you meant to say, 'he put something in it and [the barrel] weighs less than 20kg?
~Z
EDIT: if you were looking for an item that will make it weigh less, then, since weight is relative to gravitational pull, he could put a helium baloon or other artefact that rises to counteract the weight of the barrel.