I assume you are using PowerExchanges for SalesForce to do this.
It has been a while since I used it but if memory serves me right it isn't possible to work with a salesforce object when not all columns are present in the source or target in PowerCenter.
I mean sessions will fail when this is the case. If my memory serves me correct I experienced a failure of a session because a column was either added or removed in a salesforce object which wasn't communicated to me in the project.
I was working with version 10.2 back in 2017-2018.