Question RE: sales agreement contigency

[Jason Salzman]

I'm in the process of writing a counter offer for a sales agreement that was presented to me yesterday.

The way things are written now, the sale is contingent upon the buyers ability to sell their current home.

I want to write a counteroffer that will allow me to continue to keep the house on the market and give this buyer an opportunity to remove the contingency should an offer from another buyer come my way while we wait for her current home to sell.

My question is this: what am I obligated to show as proof (if any) that I have a better offer on the table?

Thanks.

Moderators, please feel free to move this post to the appropriate section of the forums if I posted incorrectly.