One of the terms you should consider putting in your agreement with the partner is a reciprocal buyout clause to make sure you are fairly pricing the piece of the business. The way it works is like this:

If one of you wants to get out of the business or sell your share, you first go to the other partners and ask them how much they will pay for your share. For example, if they say $100 per share you can refuse but have the option of also buying their shares for $100 per share this way they will make a reasonable offer because they would not want to be bought out for small amount.