[prirun]

IANAL, but here's one way to at least discuss it with him.

Let's say you own 40% of shares, he owns 60%. The code you want was developed by a single person (him) in 6 months. If you can agree that a reasonable developer salary would be $120K/year or $10k/mo, your software has a value of $60K, with $24K belonging to you already, and $36K belonging to him. So you could offer to pay him $36K for it. If you think you can hire someone to reproduce the software for less, then that would be a better alternative for you. If you can get quotes to produce the software for less, even better: you can use those to negotiate for a lower value of the software.

At the same time, if he is planning to use anything you have built together, such as customer lists, etc., you'll have to agree on a value for them and he would have to pay you 40% for those.

Does your Founders' Agreement cover any of this? If it does, it could make things a lot easier.

One of the best pieces of advice I ever received was from a lawyer who drew up a 50-50 corp agreement for me and a business partner: while you are still on good terms with each other (now!), put in writing how you will dissolve your company and part ways; because at that time, you may not be on such good terms.