If you take a rain check on something, you politely decline an offer now, suggesting you will accept it later.
(take a rain check + on something)
Can I take a rain check on that drink? I have to work late tonight.
I can't play tennis this afternoon but can I take a rain check?
Thanks for inviting me to dinner but can I take a rain check?
I'll take a rain check on that drink tonight, if that's all right.
You can also say:
I won't play tennis this afternoon but can I get a rain check?
Take a raincheck means to do something later. For example, if someone invites you out to dinner but you are busy that day then you might tell the other person - “Can I take a raincheck?” In other words, Can we reschedule it for another time?
A scene from Jag: