No Your Wrong
No Marivhon, you're wrong in your assumption. Stanislaus agreed to help you rebuild the temple if you got him a Knight. But he didn't agree to do it for free. It was going to take 20000 gp, a fraction of which (about 25%) you've already paid.
But he's pretty excited about getting the Knight, so he's basically thrown in 10000 gps of his own. Meaning you have a total of 5000 gp left to pay off before all the construction can be bought.