Deal structure when there are significant valuation differences

I am currently considering an investment in a firm owned by an individual who has a deep expertise in his industry. The person’s business involves both consulting services and a software that he markets to his clients. I am looking to acquire significant equity in this venture. However, there’s a 2-3x difference between the owner’s valuation of the business and our valuation. We are evaluating various options to structure our deal in this situation where if the owner’s forecast plays out, he’s rewarded for it and if it doesn’t, I do not end up overpaying. I would appreciate some advice on deal structures involving equity, debt or convertible debt that can be used in such situations where there are significant differences in perceived valuations.

Thanks