Should I pay short sale negotionation fee

I offered a short sale, bank counter back to us last week, add $2000 to sale price and ask us to pay $3500 negotiation fee. House price is average market price, $365,000, can rent for $1800 per month. Loan is $150,000. This is in Southern California. Initially, the agent put down "Buyer responsible for negotiation fee" on the offer contract and told us that we will get credit from the bank, we don't need to worry about it, but now bank refuse to give credit, agent then pass all the negotiation fee to us. I told the agent(Dural agent)I will pay the $2000, but not pay $3500 negotiation fee because you told me I will get credit and that's just the way how it's been processed. Then the agent replied to me that we have to pay the $3500 fee and they can try to negotiate with bank not to add the $2000. Is this reasonable and should I accept this counter offer or walk away? Trying to be a landlord, 1st time buy short sale, no experience, any input is highly appreciated, we need to decide by tomorrow morning.