Your question is a great example of why so many experts recommend planning for the break-up of a partnership before you actually start the partnership. These plans are called buy-sell agreements, which can help preserve the continuity of business ownership and to ensure that the partners are treated fairly, as well as any new partners.
These agreements work only when you agree on a formula before there are problems, before somebody needs to go.
Now, however, you're stuck with negotiation. Find a good attorney, or perhaps a mediator who works with an attorney you both trust. You're going to have to work it out. Look for expertise and experience in small business and your type of industry.
Related: Exit Strategies for Your Business
Related: A Guide to Building an Exit Strategy
Tim Berry is the chairman of Eugene, Ore.-Palo Alto Software, which produces business-planning software. He founded Bplans.com and wrote The Plan-As-You-Go Business Plan, published by Entrepreneur Press. Berry is also a co-founder of HavePresence.com, a leader in a local angel-investment group and a judge of international business-plan competitions.