Such are the dynamics of business travel and financial markets that it turns out that American Express, which has been a vocal critic of Concur’s open booking initiative over the years,¬†actually wields a 15% ownership stake in Concur, and has agreed to vote in favor of SAP’s $8.3 billion acquisition of Concur.

Under the terms of a Concur-American Express voting agreement, published in a Securities and Exchange Commission filing¬†a day after the Concur-SAP merger announcement, American Express agrees to vote its shares in favor of the merger, not to sell them, and to oppose any other “extraordinary corporate transaction other than the merger” in the event that another suitor enters the fray.

The agreement is good through February 18, 2015, by which time SAP and Concur expect that the merger would have received regulatory approvals, and the transaction should be closed.

While Concur has been a maverick in debuting its TripLink solution, which enables business travelers to take advantage of their companies’ negotiated rates when booking on consumer websites such as United.com, Avis.com and Budget.com, conservative forces in business travel, such as American Express Global Business Travel, have opposed the move and sought to entice employees to book through prescribed corporate booking tools instead.

But that strategic opposition in the business travel marketplace hasn’t stopped American Express from investing heartily in a growing company, Concur. With SAP paying Concur shareholders $129 per share in cash — a 20% premium over Concur’s closing stock price the day before the merger announcement — American Express stands to make a windfall on the merger.

Photo Credit: American Express Travel Related Services agreed to vote in favor of the Concur-SAP merger. American Express