Post Sale Price Adjustment Advice / Attorneys with Litigation Experience

Has anyone that purchased a business with contracts in place went through a mediation or repricing of the business after finding hidden business practices that were fraudulent and against contract guidelines some time after owning the business?

In more detail, business was purchased with contracts in place. Through an audit with the customer, it was determined that the business was not following contracts. Since a big portion of the profits (which the price was based upon) were made through a process that was not contractually valid and no longer allowed by the customer, we are trying to pursue a price adjustment with the seller who holds a seller note.

Has anyone been involved in a similar situation or have any advice? Also any recommendations of any attorney with relevant experience would be appreciated!