I was stuck at this problem as well, but finally I figured out i guess, if you want to allow to decrease the qty on picking while decreasing on po line, update _prepare_stock_moves  method at addons/purchase_stock/model/purchase.py at purchase.order.line model:

        if float_compare(diff_quantity, 0.0,  precision_rounding=self.product_uom.rounding) > 0:
            po_line_uom = self.product_uom
            quant_uom = self.product_id.uom_id
            product_uom_qty, product_uom = po_line_uom._adjust_uom_quantities(diff_quantity, quant_uom)
            template['product_uom_qty'] = product_uom_qty
            template['product_uom'] = product_uom.id
            res.append(template)
        return res

Remove that        if float_compare(diff_quantity, 0.0,  precision_rounding=self.product_uom.rounding) > 0: condition, it should work. Thanks

Hi @Bishal what is the value of diff_quantity?