Your portfolio is how you show your potential future clients what you can do, what's your style, the types of businesses you've worked on in the past, results (if possible), etc.
You don't work for free. Ever! (OK. Almost never but certainly not to get a project)
In your proposal you list out the various options: 3 Designs - $X; Implementation of a design - $Y; Also include how many iterations/revisions are included and also what each extra revision costs.
After your proposal is accepted, get a contract in writing, listing out the deliverables, payments, terms, etc.
If it's your first time working with that particular client, get a deposit upfront. (Also, make sure you have a point in your contract that clearly states all IP is/will be transferred upon full payment)
Your predicament is something I've faced early on in my career and something a lot of other freelancers face on a daily basis.
The short answer: You need a shift in your thinking and how you approach a proposal.
The long answer:
That's how you can hedge your risk and protect yourself to some extent.
However, keep in mind that you also need to properly present your work. Like and dislike are subjective in nature. Clients often don't know what they want. It's your job to walk them through your process, explain the what, the why, the how, etc.
Make sure you ask for references of what the client has seen that they think would work for their brand/image.
Last but not least... be ready to walk away from a project, any project for that matter, if something feels off. If you get a feeling that the client is trying to take advantage of you.
Best of luck,