If available, offer them a raincheck.

Issue the customer a raincheck for the item that is out of stock.

At my store rainchecks are good until you buy the product. At some other stores I hear they put expiration dates on them. But just ask the manager who is writing the raincheck for you.

The word 'rain check' is a (separated) compound noun, a noun made up of two words that forms a noun with its own meaning.

Take a rain check means to reschedule an apointment or plan.

It basically means to try or come again soon. Hereis aconversation example:

Customer: Do you carry these shoes in size six?

Clerk: Usually, but we haven't gotten a shipment yet. Take a raincheck on saturday.