I managed to do it successfully the way @Bojack said and paid 1,97 CHF commission to convert my CHF to USD.
Basically I’ve put the full amount I wanted to convert and pressed “preview order” to check the commission. There, it said the commission will be 1,97 CHF for the amount I wanted to convert, so I modified my order to subtracted this commission (tbh, I subtracted 5 CHF to be on the safe side, but then 3 CHF stayed in my account).
This was the way I’ve found to not have negative CHF balance.
PS: The 20 CHF fee was on the test account while trying to convert 1 million.