Salesforce batch class to update unrealed object field with rollup summary field.

Need help to develop logic for batch class for below scenario.

I have one custom object Invoice and another object Invoice details which have master detail relation with Invoice. In Invoice there is one Rollup summary field Total Value with amount field of the Invoice Details. In Invoice object there is another field blanket number. Note : One blanket number against the multiple Invoices. i,e blanket number 123 --- invoice 1 blanket number 123 --- invoice 2 blanket number 123 --- invoice 3

Have another custom object Invoice details and there in one text field blanket number which can be filled manually by sales person and another field Total amount which would be populated by trigger. i,e if sales person enter blanket number 123 then total amount would be addition of invoice 1, invoice 2 and invoice 3 total value(addition of rollup summary field) above logic should be implemented by trigger and working fine.

Now the requirement is if new invoice with invoice item will come(by interface) with the same blanket number then Total amount field of Invoice details should be updated with addition of new rollup summary field value (old value + new rollup summary field value). i,e if old value id 1000 and new invoice rollup summary field value is 500 then Total amount field of Invoice details should be 1500.

Hope this requirement is clear. I appreciate any help to write batch class for these.

Thank you in advance.