Why would Dumbledore ask Snape for something in exchange to saving the Potters?

Dumbledore might have been a jolly old friendly man on the surface but his history tells a different story (Him wanting to subjugate muggles together with Grindelwald for example). He had little issues with using others as pawns in the game, and such less then ideal traits never vanish completely no matter how much he has changed(Even near his death he still used Harry as a tool against Voldemort).

And when he made the deal, Snape was in a position to give something Dumbledore needed, information. Because at the time it seemed that Voldemort was on a winning streak and Dumbledore and his allies placed all their faith in the prophecy of the boy who would defeat the Dark Lord. And although protecting the child and it's parents were in the best interest of Dumbledore anyway, Snape was in a state of fear of seeing the woman he loved die and therefor easily manipulated. Dumbledore, being the less then perfect person, exploited this moment of weakness to turn Snape in to a spy (He was one of the rare few in Voldemort inner circle) giving an edge in the war they desperately needed.

So what Snape gave in return is volunteering to become a double agent against a person who would most likely give the most horrible death he could imagine to a traitor...not a light risk to take.