The party in my campaign did not pay attention to the verbiage being used when asking for information from an NPC while searching for clues to complete a quest. The party agreed to return with payment later for information now as they did not have what the NPC wanted at the time. Upon stating they would return with the payment the NPC stated "Contract accepted" and provided the party with the information needed.

It has been almost three ten days in game and the party has forgotten to return with payment. How would you go about reminding them? It was a simple debt to Xoblob the Gnome in Waterdeep, I just tweaked him a bit and made him more old school Fae. All they needed was something purple that he did not already have. 

Thoughts:

• Have one caster party member roll 2d4 each day after a long rest. they would forget two cantrips determined by roll order of their spell list. 
• Each night they could roll a history check to determine if they remember the payment. If success I tell them, if not....groundhog day, next morning rinse and repeat.
• Maybe have all the parties color shift to purple so it jogs their memory but now they cant tell if it is really purple or not. 

Other ideas?

Well - Xoblob doesn't seem super powerful on his own, but I only googled him and looked briefly at the first page that popped up.

To my mind, Xoblob waited a reasonable amount of time, then sold the debt to a fae debt collector. Now, the debt collector doesn't mess about - at all. This can be every bit as weird, or painful, or horrifying as you please. If feeling generous, the debt collector would simply come to the characters dreams each night, and remind them 'oh, by the way, you owe me money'. 

Then, if they didn't fairly quickly settle the debt - with substantial interests - then they'd regain nothing from resting: No spell slots, no HP, no nothing. 

Sanctions to be lifted when debt has been settled in full.

Bad dreams and exhaustion levels could be good.  Only remembering a few snippets of the dreams like "isn't there something you forgot"

"Debts must be paid"

" I WANT MY TWO DOLLARS!"  (Sorry Better Off Dead quote)

I'm a big fan of fey taking names. You could give them the task of taking a name from a powerful intelligent creature, whether it be a demon, dragon, wizard, etc. Basically force them to either trick or bargain with a powerful monster instead of just kill it and bring back its right

If you have a pet collector party like me you could also ask them to get its firstborn. Finding a mother dragon and taking its baby, only to have to give that baby up has the potential to be heart-wrenching, https://omegle****/ but also gives them the potential to level up enough for them to be able to challenge it. And by that point the Fey may have fundamentally changed the dragon

The problem with being too subtle is that it runs the risk of confusing the players, leading to them having no idea what you're talking about.  I'd just say that they have a party-wide nagging feeling that they've left something unpaid.  Give them intelligence or wisdom checks to successfully remember.  And if they keep dithering on it, have them all suffer from the effects of the Bane spell that can't be removed until they actually resolve the debt.