JJM wrote:If offered the position and I don't take it I feel it could ruin my chances of any position.
So, let's look at this in a slightly different way. Let's say you had two different offers you were considering. You're going to have to turn down one of them, aren't you? Is that going to "ruin [your] chances of any position"?
As long as you're professional about it, it's just business. People decline job offers all the time, and for various reasons. As long as you don't do something foolish to burn your bridges, your decision is likely to have zero impact on your future employability.
JJM wrote:The problem is that I really want to have my proposal, that is almost finished, accepted and work on that. That would be my first choice. However, because it will be my first research proposal, there isn't guranteee it will be accepted though I am trying my best to keep a positive attitude.
I'm a little confused here about where exactly you would be working on your proposal. Are you planning on continuing your current post-doc? And is this something that would be time-critical that you would need to start on right away? Or is it something that you could save for your first "real" position?
Remember, post-docs are temporary positions. Your goal should always be to get out of them as soon as possible to start your "real" career.