A financer is someone who provides money for a particular undertaking. A financier is a person or organization whose business is providing, investing, or lending money. In other words, a financier makes a habit of financing, while a financer might do it only once or occasionally.
Some dictionaries don’t list financer—and your spell check might disapprove of it—but this is a mistake. As long as finance is a verb, and as long as some people who finance things aren’t financiers, there will always be financers.
He was a gunrunner and a financer of such poaching attempts. [The Times of India]
Billionaire financier George Soros is warning the United States risks something similar to the eurozone debt crisis. [AP (article now offline)]
Mobley … is the sole financer of the proposed Summit Medical Compassion Center. [Providence Journal]
The announcement comes a day after the World Bank (WB), the main financier of the Nairobi Urban Toll Road Project, announced it was pulling out. [The Standard]