Checking the Macquarie and Australian Oxford dictionaries there is no mention of financer, but financier is included in both dictionaries. Checking online dictionaries including the Merriam Webster (American), Oxford (UK) and Collins, none include financer. The only online resource to include financer was Wiktionary, but as Wiktionary isn't a reliable authoritative resource, you need to take care putting any credence on whether or not it is a correct spelling.
Microsoft Word's spellchecker marks both financer and financier as correctly spelled words, which is one reason people may inadvertently use financer and think they've used the correct spelling, when they really should have used the word financier.
A check of Google for Australian sites (sites ending in .au) produces 872,600 results for financier and 42,700 for financer.
As there is no reference in any of the authoritative references to the spelling financer, this would tend to lead us to accept financer is a spelling error and financier is the correct spelling.
Kelvin Eldridge
www.Australian-Dictionary.com.au
The preferred Australian English spelling.