Recording (and reimbursing) volunteer purchases

Our nonprofit uses an accrual system - both for accounting and in real life. Many organizational expenses are paid by volunteers and reimbursed after the fact (when a grant comes in, after event income is collected, etc).

We need a way to log these expenses against a supplier, while tracking the person who fronted the cost and whether they’ve been reimbursed in full.

From my understanding, typical HR > Expense claims fall short since there’s no way to to link them to a purchase invoice (and the ERPNext goodies that come with it).

Thinking one way to handle this (what we did in QuickBooks) is to create an account for ‘Volunteer Expenses’ from which the supplier is paid, but unclear how I can track what volunteer paid the expense or their reimbursements.

Any ideas?