vendre la peau de l’ours

To count on a success before it's actually happened

It's a warning against celebrating or spending winnings before you've actually got them, don't bank on an outcome that isn't certain yet.

On n'a pas encore signé le contrat, alors arrête de vendre la peau de l'ours.

We haven't signed the contract yet, so stop assuming this deal is already done.

It comes from La Fontaine's fable 'L'Ours et les deux Compagnons' (1668), about two men who sell a bear's skin before they've actually caught the bear. The moral spells it out directly: never sell the bear's skin before it's on the ground.