When I joined ASCS one of the things to learn was the relationship of ASCS and CCC. Essentially the Commodity Credit Corporation was a way for USDA to put on another persona, a corporate one, allowing it to bypass the annual appropriations process.
It had the most impact for me when we were trying to impact new farm legislation and were on a very tight schedule. Lew Calderone, the head of printing, would ask whether the program specialists could justify the rush job as fitting under the CCC's responsibilities. When the answer was "yes", he could bypass requirements to go through the department and GPO and send the work to a printing contractor. (At least, that's the way I remember it.)
I was also aware that CCC and ASCS had separate inventories of personal property, depending on whether the item had been bought with appropriated funds (ASCS) or corporate funds (CCC).
The agency's ability to switch between ASCS and CCC personas was the envy of other agencies,like SCS and FmHA.
In the mid-80's through into the 90's ASCS and USDA began to use the CCC authority more widely, which is where the agency came to grief. As I understand it, the procurement and automation people used CCC funds to buy a lot of computer gear. What's worse, the computer projects didn't work out--success might have had a different result
Anyhow, the bottom line was the House Appropriations Committee put restrictions, tight restrictions, on ASCS and USDA on their spending, including spending of CCC money. As far as I know those restrictions remain in the current law.
This leads to my advice to Trump: any effort to reprogram money to build your wall runs the risk of stepping on the toes of the appropriators. If that happens, and I'm sure DOD will try to avoid touching anything in the districts of the members of House appropriations, the committee is perfectly capable of putting tight clamps in the appropriation act.