What cash can I accept and invest?

[Andy W.]

I have a signed purchase agreement for a two unit property (to close on or before 06/10/16). Due to a miscommunication between my loan officer and I, and me not doing my homework properly, I'm going to come up $12k-$15k at closing. There's little if any instant equity, but it cashflows really well, so I'm reluctant to walk away.

Assuming I can find someone willing to make up the difference, what forms could that investment take in order to be acceptable as part of the down payment for a conventional investment loan?

Obviously a "gift isn't an option, so would I have to enter into a partnership or LLC with the investor?