I have a similar workflow set to run daily to link invoices & POs/approvals together and then move those files to a new folder. There's probably a better way to set it up, but I'm fairly new to Laserfiche and this is what has worked for me.

The "Retrieve Field Values" step is set to pull the Invoice Number field for each entry, and then I'm able to use that information as a search condition for the next step.

I added the Conditional Sequence as I was having issues with invoices linking to ALL of my PO's, instead of just the one I was trying to target. You might be able to go without the extra step, but I haven't tested it lately since everything is working as it should and I don't want to break it. (:
These are the conditions for that step:

The reason I have the workflow move the files to a separate folder is so I can keep the search pool relatively small, so it's not searching hundreds or thousands of documents at a time (especially as I'm using "For Each Entry" twice). So I have my documents save to a Pending folder when they're uploaded, and then move to their final destination after the workflow runs.
I hope this makes at least some sense. Again, I'm probably not the best person to walk you through this, but I saw that you were still waiting for an example so I thought I would chime in with what's worked for me. Feel free to ask any questions if anything is unclear and I'll do my best to help out!