My employer wants me to run a local SEO campaign. It's a plumbing business. He briefly mentioned doing the work for free and then taking a percentage of all sales I get him. I would track sales through emails that come through the website which I will be making over. I'll also add a phone number to the website that forwards the call to their number and lets me track the calls I get them. Assuming the tracking is on point, is this a good idea? Or should I just stick with a monthly retainer?