Standard SAP already provides that service when you create a sales order with reference to the quotation. When you first create with reference it will pick up the total quantity from the quotation. If the sales order is created with a quantity less than the reference quantity (i.e. quantity in quotation) then the next order that is created with reference will only pick up the quantity that is left (so in your example with the second order it will copy only 300 kg). If you exceed the quantity from the quotation it will give you a message (Message no. V1494).
It could be that the settings for the document flow and completion rule are not maintained correctly which will prevent this message from appearing.
Sales and Distribution -> Sales -> Maintain Copy Control for Sales Documents
In the copy control from sales document to sales document you can define whether or not the document flow needs to be updated. Select the appropriate source and target document then click the item folder. Then select the appropriate item category. You will see a setting “update document flow”.
Sales and Distribution -> Sales -> Sales Documents -> Sales Document Item -> Define Item Categories
In the Item Category settings you can specify the “completion rule”. For the quotation item category it should be set to B (Item is completed after full quantity has been referenced).
If for some reason the total quantity will not be referenced in a sales order you will need to reject the left over quantity to complete the quotation.
The message V1494 is hard-coded as Warning and Information message in the includes FV45PF0V_VBAP-ZMENG_PRUEFEN_VO and FV45EF0V_VBEP-WMENG_PRUEFEN_VO.
Hope this helps.