Unfortunately, it is not possible to split the quantity of line items on an invoice created from a dispatch note (in the reverse scenario, where you perform Bulk invoicing of dispatch notes from the invoice side, you can add or remove individual line items, but you cannot change their quantities).
If you perform the split at the first step (issuing/receiving goods on a issued/received order), you can then invoice the individual dispatch notes/receipts linked to that order separately. For example, you can have an order for 4 line items of 100 units each, from which you would gradually issue/receive goods in specified quantities (e.g. line item 1: 30 units, line item 2: 50 units, etc.). These dispatch notes/receipts can then be invoiced individually.