What Law/regulation Caps the Tax Credit that I Must Pay Back at $300?

[Yagents]

Recently announced repayment extension (with interest):

IRS Eases Repayment Rules For Excess Health Premium Subsidies | Kaiser Health News

 

[RayNY]

Recently announced repayment extension (with interest):

IRS Eases Repayment Rules For Excess Health Premium Subsidies | Kaiser Health News

http://www.irs.gov/pub/irs-drop/n-15-09.pdf

Official IRS notice on the topic, for your reading pleasure.

It's basically just a waiver of the penalty for late payment or underpayment of 2014 taxes owed by April 15, 2015, but only for the portion directly attributable to the APTC repayment. You must pay all other taxes, and be up to date with filings, in order to qualify. Penalty is still generated, notices are still sent, and you must reply to the IRS in writing (including a form filled out a specific way, outlined in the notice) with some specific verbiage to buy yourself an extra year to pay off in installments (with interest).