All depends on the understanding with which they provided their proposal and what level of 'work' they delivered to you.

If the work is just what they would consider to be part of their sales process then you should be okay but if they went above and beyond with the expectation of winning the final project then there are ethical questions for you to consider.

Having said that, this is how many larger firms operate. They send out RFIs and then based on the information they get back they refine and re-issue a more detailed specification in their RFP.

If you know what you're doing and are clear at the start then there shouldn't be a problem