I doubt it is metric. The metric system was first invented in 1670, six hundred years after the end of the Viking Age.

It is probably an Old Norse unit. It could be an eyrir (plural: aurar) which is 27 grams or slightly less that one ounce avoirdupois. That would make the maximum weight a viking can carry 8.1 kilograms or 17.8 pounds avoirdupois. This seems ridiculously low.

It could be a môrk (plural: merkur) which is 8 aurar. Metric weight would be 216 grams or about 8 ounces avoirdupois. Three hundred merkur would be 64.8 kg or 142.56 pound avoirdupois. Seems reasonable to me.

The next largest Old Norse weight is the pund, which is 24 merkur. The metric equivalent would be 5.184 kilograms. I doubt anyone could carry 300 of those.

So I am settling on the Old Norse weight of a môrk which is 216 grams or about 8 ounces avoirdupois.