Once the Renters Right Act becomes law, if a tenant volunteers to pay more than one months rent in advance, how can this be documented to protect against issues in the future?
If you don’t accept the offer, tell the tenant in writing and ask them to pay on the date when due not in advance, and they do so then you won’t have issues.
For any payments made in advance either reject the payment and refund it, again telling the tenant in writing when to make the payment. Or at the very least write to tenant saying that you have not either encouraged nor required early payment which would be illegal under RRA. So that what has happened is contemporaneously documented.
Good luck
If you want to accept their offer, ask them to put it in writing. You can only do this after the tenancy agreement has been signed.