I have a Reimbursable Category that I have setup to hide from Reports. If you wanted more granularity, you could add Tags for each Business Trip, Activity, or Event.
Every so often I run a manual Category Report for the Reimbursable category to make sure it’s zero. If you’re using Tags, I’d use the Tags report.
Some people use a separate Reimbursed Category to track payments, but I prefer seeing my Category go to Zero.
You could also use a Pivot table. Here’s a recent thread about Pivot Tables, although with a slightly different use case.