Waver does say in Episode 7 that he'll buy Rider some pants as a reward for taking out Caster. In the novel he tells Rider in Volume 2, Act 5, -140:41:54, that he'll buy him a pair of pants if he can defeat even one of the other Servants.
[Waver] “Your majestic achievements and your trousers are two completely
irrelevant things! Before you go out to enjoy yourself, show me what
you can! Kill at least one of the opponents’ Servants!”
[Rider] “Eh? You are quite an impatient fellow. You can engage a Servant in a
combat anytime you want, you know.”
[Waver] “So do it now! Kill at least one of them! If you do that, I’ll buy you
trousers or anything you want.”
Later, in Episode 9 of the anime, we find out that Waver caved and bought the pants for Rider so he would be able to go outside and get the water samples that would let Waver isolate the location of Caster's workshop. In the novels, this occurs in Volume 2, Act 8, -108:27:55.
[Rider] “No no. He is the one who has been of help to me. These trousers, even
he had chosen and bought it for me. Looks great on me, doesn't it!”
At the time when he was entrusted with external businesses, Waver got
stuck with buying stuff for him, and bought an XL-size washed-jeans
for Rider, which he proudly showed off.
So Waver ended up having to buy the pants for Rider early so that Rider could go out and gather water samples without looking strange or attracting attention.