Arbitrators reformation of agreement for mutual mistake exceeded the bounds of its power, as the purchase agreement limited the scope of arbitration to calculation of specific sums, and the letter of engagement's reference to threshold levels did not empower the arbitrator to revisit the agreement that set the levels.
http://www.ca5.uscourts.gov/opinions/pub/17/17-50613-CV0.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.