Nearly same topic, but I prefer to pay for the shorter jail time. Doesn't have to be a lot. Reducing itself is okay too; but I find cash drains economicly better
DROT's suggestion is more feasible in the fact that nothing has to be created, figures merely need to be changed.
Also tbh don't think it would be ideal to be able to pay to reduce your sentence, just disenfranchises new players even further - most of which leave the server after getting a 7 minute jail, the only people who can consistently afford that are the people who have been around awhile and are going nowhere.