Duplex seller issue
@Brad Ickes Always remember you never have to buy. He should need to sell more than you need to buy. On the other hand, know what you need to get the house for and if it meets your numbers and you can make a good return don't lose the deal over small things. Now that your under contract you definitely have a little more power, especially with it being on market for so long.
emphasize that it's a cash deal and if he wants to sell his home then 91k is you offer. Make sure he knows that you are already giving him a break on the reduction only asking for 4k instead of 6k and that they are code and safety issues you're talking about which will be faced by every buyer and may prevent banks lending on the property due to current code/safety issues.
There is always another deal, don't give away money on valid issues just to get the deal.