[matt_morgan]

You're not being unreasonable but that's not the issue. The issue is the leverage you have now. Your value to your partner ends with the sale. You are valuable to the new owner as the sole person who understands how the product works. Even if they only want the idea and the customers, recoding it will cost something and be cheaper/easier/faster with your participation, plus they'll need someone to support the current product for a while, or they'll shed those customers.

Convince your partner that the buyers will need you, and will only get you if you're happy with the terms of the sale. If you can enlist the buyers in this campaign, so much the better.

[ChuckMcM]

100% agree with this, and add the following caveat.

Don't be surprised if your partner "switches gears" and tries to convince you that you are being unreasonable and selfish and screwing up the possible deal (or future deal) with these "demands." Sometimes people present as friends only because it doesn't cost them to do so, but change their presentation when being asked to reduce the amount of money they may have been counting on receiving.

Be ready for that to happen and understand that your actions are not "ruining a friendship" they are merely illuminating that the friendship was an illusion to begin with.

[awillen]

100% - any smart buyer of a company is going to inquire about key employees to ensure they know who to retain. If you can be in the sale meetings so the buyer knows who you are and how critical you are, that's ideal. If they know what they're doing, they'll try to find out whether you're going to stay and what it will take to keep you. That's where you negotiate.

If the buyer doesn't like your position, they may walk away from the sale - that may upset the business owner, but it also re-establishes your leverage. If the owner wants to sell and buyers need you but won't accept your terms, then you can explain to the owner that in order for you ask for less of a retention bonus from the buyer, you'll need the owner to give you a bigger percentage of the sale price.

[designium]

I think this is a great recommendation. Before going with lawyers, I think it's important to share your vision of how beneficial you are to the project. At the same time, you must know a bit more about the sales and the new potential owners intent. If you see there is no future for you, the new owners are just there to grab users and move onto something else, that may dictate a different strategy than you are currently stating here.

[Deep-Engineer91]

I agree. Utilize your value and renegotiate the terms of what agreement so that you get a piece of the company down the line. Don't be afraid to also demand that you need to approve of the buyer whoever they may be.