First of all you are opening yourself up to a scam.
Second, if you work around AirBnB policy and rules then you won't be covered by their Host Protection Scheme.
Third: You should ask yourself if you really want a guest who cannot afford to pay in on amount.
Having said that you can ask them to make two separate reservations, at least that way you are protected. You can then, if you wish, agreed to send a special offer as they will have paid two AirBnB service fees (but so will you)