Allow non salable product for return
From https://discuss.tryton.org/t/error-in-sale-complaint-action/2317, I think we should not have constraint on returns because product may have been salable or purchasable in the past (before the return).
From https://discuss.tryton.org/t/error-in-sale-complaint-action/2317, I think we should not have constraint on returns because product may have been salable or purchasable in the past (before the return).