How to account it ?

If I am purchasing a piece at rs 100, getting 3% as incentive on achieving the target, so finally it can cost me at 97 and am selling it at rs. 98 because of prices prevailing in the market. Now what i should consider, purchasing it at 100 and selling at 98 or purchasing it at 97 and selling it at 98. Even all other taxation related matter with regards to the above particular thing.