If it's possible to post partial refund for not settled transaction with less amount, customer just returned 1 item?
No, refund are for settled transaction only.
Or if is possible create credit without TransactionID?
Yes, you can do unlinked credit
Read the unlinked credit section
http://developer.authorize.net/guides/AIM/wwhelp/wwhimpl/common/html/wwhelp.htm#href=2_SubmitTrans.4...
