I used multithreading in CSV, to create invoices parallelly, script deployed on the invoice (AS) updates 1 custom record field to some value by total/sum up with an existing value in the field, the issue is instead sum up it overrides with last value.

Based on this description alone, it's hard to follow the logic or why it's not working. You say the deployed script updates 1 custom record field to some value by total/sum. What is the script summing up? A value from multiple lines in the invoice? Also, are these single line invoices, or multi-line? I have had a lot of trouble with multi-threaded CSV imports when dealing with multi-line transactions, so that is one instance where I usually disable multi-threading.

@scottvonduhn I dont have code handy, it captures the total amount from the invoices, there is a custom record which has customer field and amount field, the amount will be updated ( get existing value and add up with the invoice amount ) and save the custom record

Without seeing the code, it's hard to say what is wrong. There may be a bug in the code that gets / sets the value.