How to update SelfService procurement lines that are coming from a catalogue?

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Hello, 

we would like to update a line that is coming from a catalogue in SelfService procurement, so that we can change the unit price. 

However, it seems that we cannot, as we get an error when trying to update the line, saying that we cannot edit lines added from a catalogue.

Is there any workaround or setup that we can do so that we can edit the price.

Many thanks, 

Mario