If the open fine is paid or payment is proven, enforcement of the default imprisonment is omitted under section 3 StVG.
The simplest way to avert default imprisonment is to pay the open fine. Under section 3 StVG enforcement is omitted to the extent that the convicted person pays the open fine or proves payment by a public document. A partial payment already made also reduces the remainder that would have to be converted into custody. Anyone who cannot raise the sum in one go should seek payment relief or instalments early, before the default imprisonment would have to begin.
What to do now: First, clarify the open amount and the way to pay it so that payment arrives in time and can be proven. Second, if full payment is not possible, set an instalment solution in motion. Third, in parallel examine whether community service under section 3a StVG is the better route should neither payment nor instalments succeed.