Partial invoice, text positon based on sales order

Hi folks,
I’m giving erpnext a second try now, but I can’t get it working for my business.

Short: Freelancer, I get an order with 2 positions say 10 hrs with xx€ fix-price and 10 hrs with xx€ t&m (means overdelivery is allowed).

No my questions:

  1. partial invoice: when I work only 5 hours in one month, I’d like to create an invoice based on the sales order, but when I do that, the amount field is read only!

  2. is it possible to have free text positions in invoices or overwrite the standard text from the article

  3. Can you mark sales-order positions for overcommittment allowed? I know in theory that would be a delivery contract but that is not supported at all

Do you think this case can be setup without rewriting major parts of the code?

best regards,