Is disclosing the algorithm a mandatory condition of the sale? If I were them I'd not give away the algorithm. I'd sell the name, branding, userbase, and let

Is disclosing the algorithm a mandatory condition of the sale? If I were them I'd not give away the algorithm. I'd sell the name, branding, userbase, and let them lease the servers until they could host it in their own datacenter -- at which point they'd have access to most of the code, but not the algorithm (if the algorithm is of special value).