Lend the shutdown?

The Federal Government seems to be obeying with rather remarkable accuracy the constitutional mandate that the government may not spend money that has not been appropriated by Congress.

I would be curious to hear from legal experts, however, what stops the government from lending money to federal employees, or just guaranteeing loans.

After all the government lends money all over the place, and credit guarantees are even larger. Is the Treasury no longer operating small business loan programs? (Honest question.) Is the Fed no longer lending money to banks, if they want it? Are Fannie and Freddy refusing to buy home mortgages because the funds to guarantee home mortgages (which it does) are not appropriated? No. As far as I can tell, Federal lending and loan guarantee programs are up and running.

If so, what stops the Treasury, from either lending money directly to Federal employees, or guaranteeing private lending. After all, the Treasury will write their back paychecks when the time comes, so these are potentially risk free loans. What stops the Treasury from just writing on a federal employees' paycheck "this is a loan against your back pay?"

Or... Social security and Medicare are still running. Can they write advances against social security payments that will be deducted from future federal paychecks?

I presume there is something stopping this -- that it is a step too clever, like the trillion dollar coin solution to the debt limit. But I would be curious to hear what the limitation is.

(HT Marginal Revolution on federal employees' other sources of financing, at pretty high interest rates.)