split the purchase order line if shipped qty is less than ordered one

For eg, we have a order of 100 ty and we just have 60 available. At the moment, we ship 60 and manually create another line of 40 and then ship that later. What users want is that when the available qty is less than the ordered qty then it should automatically create a line of 40 to be shipped later putting it in backorder.

Is it possible?? My concern is that even if it is possible, it might mean more work for finance side. Would really love to have some ideas... Thanks.