I've a vague memory that back in the early 70's there was a flap about Agricultural Stabilization and Conservation Service county offices leasing office space from members of Congress.
Possibly it was an issue raised by Rep. Findlay of IL, who didn't much like anything that ASCS did, but I won't swear to that.
Anyhow, memory suggests that ASD (Administrative Services Division) issued notices to do a survey of how many instances of this we had and requiring the leases to end. I don't remember that there was a statutory basis for the prohibition, just a policy one.
I've done a quick look at the USDA manual on property and didn't find anything. Apparently FSA has determined not to put their handbooks covering administration on the website so I haven't checked that.
Anyhow, I thought the issue of renting office space is a good parallel with the issue of renting hotel rooms from President Trump.