How to capture verbal contribution commitments: invoice, contribution pending or pledge?
Unfortunately pledges do not work with online contribution forms the way you'd like without an extension. You can use the framework for partial payments that's available in github and seen at the CiviCon in Denver earlier this year

First you'll need to create the contribution(not a pledge) on the back end. You probably can use the same concept with a checksum email if you don't want to require a login.