You entered and agreed on a commission contract with the client. Since the shifts start within the 24, 48, 72 hours or 7-day cancellation term, it’s your own responsibility to find a suitable substitute. The client is expecting someone that is on the same level as you are. In the case where you don’t succeed in finding a suitable substitute, this will have 2 consequences:
It will have a negative effect on your attendance percentage.
The client decides whether to give you a fine of € 100,- or not, mostly depending on whether you informed the client beforehand or not.
Pay attention: If you cancel a shift before the cancellation term goes into effect, you won’t be expected to find a suitable substitute.