In my experience with both agency and client I've often heard "well we have a small budget so we can only do this..." Wait, what? Is this a creative meeting or is this a funeral?
First, let me tell you what sad little emotional creatures we overly sensitive creative types are. We're a ball of fire that you walked in and pissed on before you told us hello. This kills the energy and the love of the project right off of the get-go. Secondly, it should be every agency's dream to think on a grand scale for every client no matter the size or budget. Given that the client's inspired they'll eventually find the fund to make great big expensive ideas happen too. Trust me, they want to do them no matter how frugal you think they are.
Advertising is often 10% inspiration and 90% desperation, but it doesn't need to be. We need to capitalize on the creative 'potential' of any concept. The money might only render you realistically a microsite, Facebook banners and some revised content, but the bigger idea that need be pitched passionately to the client, is that you'd love to shoot a multi-part original story that can do X,Y and Z for the brand! No matter how dull an idea may seem on that copy paper and you sit in the room filled with dry erase fumes, remember this is the moment to harness something bigger.
So you don't use it. You did have fun creating that bigger idea, and ALL ideas in their pure form have no predetermined shelf life. Therefore you simply create intellectual equity that could be transposed to a new client or future campaign. Write it down, comp it up or simply make a note. It'll always be there and it will always be yours :)