You would treat that Advance as if it was a Distribution originally. There was never a Dividend to an S Corp Shareholder, to turn around and repay an Employee Advance, but then you called it Shareholder Loan. That's really just running in circles.

I'm not sure which things you need for reference. Here are some helpful links:

https://proconnect.intuit.com/community/form-1120-s/help/shareholder-basis-input-and-calculation-in-...

https://proconnect.intuit.com/community/schedule-k-1/help/distributions-loan-repayments-frequently-a...

https://proconnect.intuit.com/community/schedule-k-1/help/basis-limitation-frequently-asked-question...

https://proconnect.intuit.com/community/lacerte-tax-discussions/discussion/how-do-i-enter-gain-on-ex...