Help!!!

Hi,
This is the situation.

A friend offers to do something for me.
Well,I would like to try to do it myself first and only if
I am unable to do,I want to accept his offer.
i.e.Use his offer at a later time.

I believe there is a saying for this?I can't remember
it exactly...All I remember is that it's something
like'keep the tag'or 'keep the ticket/token'
would someone help??

TIA,
Biju

"Take a raincheck" is one possibility. It's literally a ticket stub you get upon admission to an outdoor event that allows you in for a repeat engagement in the event of a rainout. Also used at retail for a sale item that sells out, allowing you to buy it at that price later when it's restocked.