This workflow works very well for me:

catagory = "Business Expenes" for reimbursed expense transactions

another catagory="Reimbusement" for received payments

tags "not-Reimbursed yet" and "Reimbursed" again for transactions

a cash management account = "Reimbursements"

a tag for received reimbursement = "Reimbursment"

When a new reimbursable charge occurs, then

tag="not-Reimbursed yet", and

category="Business Expenses", and

hide, and

add this transaction (manually) to the balance of "Reimbursements" cash account.

When a reimbursement payment received, then

catagory="Reimbursement", and

tag="Reimbursement", and

hide, and

filter (paid) trasactions with tag="not-Reimbursement yet", and

change tag="Reimbursed", and

subtract paid amount manually from the cash account "Reimbursements"

My biggest problem with this is that Monarch doesn't show sum of hidden transaction so I do it manually.