Part 1: News Reports
FoxNews: http://www.foxnews.com/politics/2016/08/24/us-paid-iran-1-3b-two-days-after-400m-cash-transfer.html (Archive link here: http://archive.is/czZ9V)
CNN: http://edition.cnn.com/2016/09/07/politics/iran-cash-payments-congress-hearing/index.html (Archive link here: http://archive.is/ol0lU)
Part 2: Explanation
Obama State department wired $1.3BN of funds on Jan 19th 2016 after the original $400M of untraceable cash transactions to Iran (The $400M is referred to as the "Ransom payment").
Funds were wired suspiciously in 13 separate payments of $99.999M which totaled $1.3BN
Part 3: Importance of $100M Mark
This is a general limit of funding and appropriation that does not need congressional approval.
I don't have a hard link to the specific parts of US Code, but two specific House resolutions point to this limit - both suspiciously worked on in 2015 as a way to bring more accountability.
HR 427 (https://www.congress.gov/bill/114th-congress/house-bill/427)
Why this is highly suspicious and probably illegal. $100M is a limit that needs congressional approval, but splitting the larger transaction into smaller ones that escape scrutiny is an illegal act for a private company under Sarbanes-Oxley and other Generally Accepted Accounting Principles. Unsure if it's illegal for the gov't , however it's very suspicious and unethical.
Part 4: Funding and Transaction Logs
The funds were taken from a obscure fund from the US Treasury called the Judgement Fund. Often used to pay to avoid litigation
Fund Information: https://www.fiscal.treasury.gov/fsservices/gov/pmt/jdgFund/judgementFund_home.htm
Excel Log of transaction (may expire): https://anonfile.com/VaM6rbbab8/Judment_Fund_Payment_Report.xls
Part 5: Do-It-Yourself: Transaction Logs
URL: https://jfund.fms.treas.gov/jfradSearchWeb/JFPymtSearchAction.do
Enter the following information using this image: https://sli.mg/QBloiM
You will now see the 13 separate payments made