> Your position is stronger than you think. A buyer will want your cooperation as a top priority.
To emphasize this: as the sole developer, the buyer is going to want to retain you (at least for 6-12 months, depending on the complexity of the software). The buyer's tech team is not going to want to reverse engineer software that's 20 years old. The owner of the company knows this if he's talked to any potential buyers so far (and if he hasn't, will learn it.)
> Best to be calm. This is your strong card, but keep it low key.
The owner may resist your initial ask. If you at any point see red and are tempted to raise an ultimatum, step away from the send button / tell him that you'd like to continue the conversation later. The worst thing you can do is back him into a corner where his ego gets involved. You may well run into conflict, but better to leave him aware that you disagree until you pick up the conversation again than to get into a heated dispute (that will incentivize him to think about a solution in the meantime too).