Surely you mean "if it doesn't rise then don't have bought it."?
(Will Rogers, giving advice on buying stock.)
In Douglas Adams' TimeTravelGrammar, I think it's "Iffate it not rising, boughten it,"
Which book is that from?
Dr Dan Streetmentioner's Time Traveller's Handbook of 1001 Tense Formations
We hear about it in the Hitch-Hiker's Guide to the Galaxy universe (I'm spotty on the exact book) - I was just making the previous example up, using some of the syntax I remembered. Then again, since he probably made up the words on the spot himself as he typed it, this is all a complete waste of time. Is-being-forthwithenate.
Heinlein postulated a similar convoluted grammar in GloryRoad? (early 60s).
"Primer" is another relevant work http://www.imdb.com/title/tt0390384/