I doubt anyone can give you useful feedback or advice based on the information in your post. It really depends on what your contract with the company states regarding relocation expenses. How it is worded, the exact specifics of what happened with your boss, etc. Since HR has agreed to allow you to apply for internal positions, you might have to take one if a suitable one is available. However, without knowing exactly what your contract says, it would seem to me that if there is no internal position available that you are qualified for, and your problem was not one of poor performance, and you have no options left but to apply to other companies, you might be off the hook, or partially so, on the reimbursement.

If the terms of your contract relating to the relo expense reimbursement are not clear, you need to see an attorney who specializes in employment law. It's worth an hour's worth of legal time to have him or her review the contract and interpret it for you. It's good to establish this relationship in any event, because if you do go to another company and there's a dispute regarding this matter, many times a well-worded letter from the attorney can resolve the situation before it becomes overly involved.