How can I set a script to re-lookup only in the current found set?

Actually what is the best way to Create a new invoice record for every existing costumer record and do a (re-)lookup?

Relookup always works on the found set. And, why would you need to do a relookup? Relookup what, from where?

As far as creating the invoices, just import the Customer IDs. If you're wanting to duplicate current invoices, then duplicate them in a loop, and clear the non-duplicating fields; or, if there's a lot of those fields, then set the current ID into a global, create a new record, and put the global in the ID; then omit the original record (or both records, then find the new ones).

This isn't very clear; but then it's not clear to me what you're wanting to do.

I am managing an Office/Apartment complex. Every month I need to invoice all the current tenants, many of the invoice charges items like rent are recurring, I need to find a way how to create an invoice then lookup the charges (one-time and recurring) for that tenant. Every month I need to create a new set of invoices for all the then existing tenants.

I would think you would just Import the entire Rentals table into a line items table every month, with the Rental ID and the rent due; and the ID or person responsible for the rent. I don't know about other charges. Those would come from another small table, but would also come into the charges table, to keep all the money in one place.

You could use a loop, but when you're dealing with a large group of records, import is the most direct. Mind you, Import has been a bit buggy in 7.