When creating/updating an opportunity in a workflow, it's not currently possible to update a numerical or monetary field using a bracketed value. You have to type a number. This is actually a workaround for not being able to update Opportunity fields via a LeadConnector zap action (have to update contact fields via Zap, then update the opportunity in a workflow). But cannot update the opportunity properly in the workflow without being able to dynamically populate numbers like {{contact.30_margin}} (see image attached).