Preposition has two definitions: (1) a word or phrase used to relate a noun or pronoun grammatically to another part of the sentence, and (2) to position in advance.
Proposition means (1) a plan or offer suggested for acceptance, (2) a matter to be dealt with, and (3) to propose a private bargain.
Examples
Preposition
He also instructed all city and municipal DRRMCs to prepare all necessary emergency equipment and personnel, and preposition them if needed. [Sun Star]
She has American and English models, but her parents are intent only on marrying her off (” off” being a particularly repugnant preposition). [Wall Street Journal]
The World Health Organisation, present at the Tunisian and Egyptian borders, has prepositioned drugs and medical equipment there. [Reuters]
Proposition
Ms Gillard is due to appear on Insiders this morning, where host Barrie Cassidy could proposition her. [Sydney Morning Herald]
The complainant said she was walking on the pathway when a man emerged from some trees and propositioned her for sex. [Ithaca Journal]
It certainly is a realistic proposition but it’s not as straightforward as driving into France. [Telegraph]