What were the agreements again? After a while you often don't remember exactly. It is also good to clearly establish the rights and obligations on both sides. That is why it is always good to put this down in writing.
The volunteer contract can include which expenses the volunteer makes and which are reimbursed. If the tax authorities ask for it, the agreements made can be shown.
Volunteers can incur damage during their deployment but they can also cause damage. It is important that you can prove that there is voluntary commitment. You can do that with a volunteer contract.