Well that stinks! (sorry, couldn't resist :-)
Seriously though, liberally rubbing in some Brooks Proofhide on the top may loosen much of the offending molecules and offending olfactory updrafts. Wait an hour or so and wipe off as much as you can with several paper towels. Hopefully much of the offending sweat will come off along with the excess Proofhide. Might try this twice,...for stubborn cases.
There is also a leather "cleaner" I've used called "Meguiar's Gold Class Rich Leather Cleaner/Conditioner" that might do the job (Since 1901,...so it's been around a LONG time!) I've never used it on my bicycle saddles though,...just on several leather binocular cases and on my Mazda MX-5 Miata 2 seater leather seats. In you case, I'd probably try this first. I bought my 14 ounce bottle at a Pep Boys or Auto Zone Store several years ago.