I agree with the rest that think it would be ok to take cash for repeat customers. I also don't think it's in violation of their terms of service (at least I haven't seen any rules about repeat stays). While I do see the point made that we sort of owe Air for the service they give us hosts and the fact that the guest is yours in the first place because of Air, I do think that repeat customers come back because of YOUR efforts to be a good host. So in that sense, you've gone out of your way to ensure the guest comes back and Air had little to do with that.
I've done conversion to cash for good guests who have asked me during their stay if they could come back in a few days. However, sometimes there are guests who come back months/a year later and I do ask them to still go thru Airbnb. I only do this to ensure payment for the dates I block off because I rarely get last minute bookings, so no-shows and cancellations are really sucky for me. In your case though, I guess it doesn't apply.
Something to think about also is the advantage of setting up your own PayPal or Payoneer account for collection in case future repeat guests want to pay via credit card. This could also allow you to get paid in advance, or if you are not comfortable with that, have guests pay half through your payment channel then the other half in cash upon their arrival. Have not really researched on the fees involved there but I thought it would be worth looking into?