The reason you do not have to take the sale as income when the contract is made is that the terms of the contract are not yet "completed"-------and won't be until the basis is set.   Upon completion of the terms, the unknowns are gone, and the contract can be ajudicated, so the sales contract is then income.

In order for any transaction to have tax implications, it has to be completed..............the exact terminology escapes me right now, but do a google on "completed contract for tax purposes", or something along those lines, and you should get details.