@Klaus Interesting, in the U.S., or at least here in Connecticut, this would not be allowed. For instance, my older brother was divorced and based on his income, it was determined he would have to pay a certain amount in child support.
While the pay was 'ok enough' the schedule was very inflexible. So he went into business for himself. Ultimately he ended up making more, and having the closer to the schedule he'd hope for, but it took a couple years.
The court would not lower his weekly amount due, since his separation from employment was voluntary. (even if it was to ultimately lead to something much better) At one family court hearing, he was placed in cuffs and put in a holding cell. Asking the judge, "How can pay in I'm in a cell?", he was told "We find that within 48 hours, almost everyone is able to come up with it."
Well, the judge was right. His family came up with the balance...Once again, in this case, it eventually all worked out, but around here, if it is determined you can work and earn the money for your support order--you are obligated to do so.