Well the fairness issue is one I can understand even if I think you need to put yourself first in this instance. However, at least in the UK, I've never heard of a contract of employment that doesn't include a "notice period" even if the contract is for a fixed term? Like it's often somewhere between a week and two months depending on how long you've been there/what the job is, etc.? I don't know how it works in Germany but in the UK, you'd be legally entitled to back out of the contract as long as you offered them a fair notice period so they could hire someone else.