Shortfall on sale- ac* bank

I bought a house in 2004 for 174k.

It is currently rented out. The tenant wants to buy it from me.
Balance on loan circa €128k. Repayment €963 rent received
€550. House is in good condition but possibly only valued at € 85000. This leaves a balance of €43k. I'm inclined not to sell it because of the shortfall.
However I put €425 into mortgage each month plus other expenses of letting.

The mortgage is with Ac* whom are departing Ireland but leaving their loan book with a loan management company. How should I approach the bank with this scenario.

Scenario 1) sell for 85k do a deal with the bank to share the burden of negative equity.
2) Continue renting it out and sell in a few years.
Hoping the capital repayments and a rise in value erodes the negative equity.

Selling is your decision , provided Acc agree.
Scenario 1) Why would Acc do any deal on negative equity on an account that is being paid? They have enough (bad) acs to negotiate without looking to help you.Unless your situation disimproves there is nothing in it for Acc.
2) Looks like prices are lifting , so wait seems your option.

Acc are leaving but are using a management company to collect as much as possible.

Since you are able to pay your due debt there is no incentive for Acc to do anything.

Sorry , but that is how I see things.