@Antonia Powell if your partner has cold hard cash, have them deposit it ASAP. If you're going for traditional financing, NO lenders will accept it.
For primary purchases, you can get around it by asking family for gift funds, then "gifting" the relative the cash later, so the lender can source the funds.
But for investment property loans, cash gifts are not allowed. Though, if you live there, it would be a primary.
If it's FHA you can do it with 3.5% down but you'll need a full month of bank statements from the family gift donor.
If it's conventional, you can get away with a withdrawal slip, but the down payment is 15% for a duplex and 25% for a 3-4 unit.
For 2-4 units with a non-occupant Co-borrower (aka your partner) you need to provide 3% of the down payment from your own funds.
Lastly, it might be tricky to have the realtor as a non-occupant Co-borrower. That's a new one, I'd have to look at the guidelines again for that.