Mass. They wanted to know if they could buy the rights to use one of my images off the web in advertising. He said he'd call me tomorrow with an offer. Stuff like those blue dogs that Xerox uses everywhere net the artist quite a wee bit. If it's to be a key image for an ongoing advertising campaign you might ask a small fortune too. Publicity or prestige benefits might be a consideration to keep in mind.
Every situation is different and requires a bit of thought.
If it's minor, keep it simple, even verbal, and count on good will. If it's extensive, consider a contract. The operative factor with contracts is time. If he makes a fuss and the ante is up from the Mass Financial people you might consider buying the painting back. I got around a non-understanding owner once by painting a somewhat similar but dedicated painting for the advertiser that worked better anyway.