In the movie Shrek 2, when King Harold and the Fairy Godmother are in her "car" she says
"we had a deal, and I assume you don't want me to go back on my part".
What was the deal?
*unmarked spoilers to follow*
Canonically it isn't stated.
However one can assume that since the King was actually a frog originally and he fell in love with a human princess the deal was to turn Harold into a human prince so he could marry said princess.
The deal was - the Fairy Godmother would help King Harold (i.e. the Frog Prince) to marry Lilian (i.e. Fiona's mom) via Happily Ever After Potion. And in return, the King would let the Fairy Godmother's son Prince Charming marry Princess Fiona.
That was the plan all along. That once Prince Charming grows old enough, he will rescue Fiona and marry her. Quotes as below showing the disappointment of Fairy Godmother -
King Harold: Charming! Is that you? My gosh, it's been years! When did you get back?
Charming: (Irritated) Oh, about five minutes ago, actually. [Raised tone] After I endured blistering winds! And scorching desert! Climbed to the highest room of the tallest tower--- (Cut off by the Fairy Godmother)
Fairy Godmother: (Concerned for Prince Charming's upset tone) Tut, tut, tut. Mummy can handle this. [to King Harold, using the same hostile tone Charming used earlier] He endures blistering winds and scorching desert! He climbs to the highest bloody room of the tallest bloody tower! and what does he find?! Some gender-confused wolf telling him (Harold tries to explain but gets interrupted) that his princess (Harold attempts to explain again) (The Fairy Godmother's voice is more settled now) is already married!
And then again from the fast food joint scene, the threatening becomes even more clear,
Fairy Godmother: We made a deal, Harold, and I assume you don't want me to go back on my part.
King Harold: [sighs deeply] Indeed not.
Fairy Godmother: So, Fiona and Charming will be together.
King Harold: Yes.
Fairy Godmother: Believe me, Harold. It's what's best. Not only for your daughter...but for your kingdom.
Courtesy - wikiquotes.org and moviescript