I'm not saying go into details right this minute, which is why the modification I made to your vote give a fairly generic while still defining scope and category for the payment.Its not mentioned because its both tacky and time consuming to adjudicate right this moment.
Not to mention potentially insulting if the elemental chooses to look at it as a gift and mark of its generosity towards someone its hoping to cultivate as a friend/ally instead of a business transaction, and you then offered payment.
Sometimes favors are just favors that people remember and repay in kind down the line.
We havent formally acknowledged debt for Harry Dresden bailing us out during the Splattercon events.
Or for coming to rescue us from Splattercon, or running interferemce with the White Council. Or saving little Harry's life from Kravos, or his sanity from Lasciel. It doesnt mean the debt doesnt exist.
We arent Fae. Neither is the entity we are currently dealing with.
Not everything has to be boiled down to immediate numbers and calculated measures of value, and certainly not immediately.
EDIT
Maybe its a cultural thing. But this is not something that demands immediate adjudication.
Its part of the soft exchange of favors that lubricate society among the non-fae.
The fae have their own rules.
We don't know this guy really; he isn't our friend, business partner, or subordinate. Soft favor trading is a bad plan with a stranger. Especially one who shares as little in common with us as this one; it's better to be explicit to avoid complications.
The little interactions that grease the wheels of society are context based, and we haven't established much common ground with him yet.
A one sentence acknowledgement of the bill that confines the sort of favor owed isn't going to break our time table.