FWIW, the company I worked for until Feb 2019 had a "negotiated fare" agreement with DL, and required booking through Concur. The fares there rarely showed any difference from retail, buuutttt...... in talking with the internal travel dept, I learned that the negotiated fare difference for our company was in the form of a % rebate based on total sales. No idea if this was standard or just us, but we were encouraged to book personal travel through the concur portal for mainly this reason (secondary reasons likely would include the smidgen of difference in total volume that would create in the value of the overall contract).