Scubaboy, I can see whay your customer may feel a greivance against you, and as the loss of his goods was not of his making but happened when you had charge of them he will want restitution from you. He may well think that by witholding payment legitimately due to you for other matters may speed up the settlement, but the payments that he is witholding are not related to his loss and therefore should not be witheld.
However, as this is a civil matter you will have to pursue the customer under breach of contract through the Small Claims Court, and the customer will almost certainly counter claim against you for the loss of his property, even though it may have been the subject of an insurance policy. So, to issue against the customer will only worsen the stalemate.
A far better solution would be to prod the insurance company for a swift settlement, which in truth is what both of you want.