You entered an assignment agreement with the client. If you were not able to cancel your shift before the 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, your client has the possibility to issue a no-show. A no-show could have 2 consequences:
It may have a negative effect on your attendance percentage if your client issues a no-show.
The client can decide to demand a compensation fee of € 100,-. Informing your client upfront might prevent this, but you will also have a chance to dispute the fee after the shift. You will have a one-time option to come with a counterproposal in which you share your reasoning for not making it to your shift. It’s still the client that makes the final decision whether to maintain or waive the fee.
Pay attention: If you cancel a shift before the cancellation term goes into effect, you won’t be expected to find a suitable substitute.