A bird in hand is better than two in the bushes. You can just say you are sick by calling them on the last minute. You are just going for the first interview. The entire process may take some time. You will still end up temporary working for the company that had firm up the job with you. So, it will come a time you decide to go and be face with the same scenario.

Jnls,
In that case I think you can go for the interview and start negotiating salary. They already know what you are capable of from your internship period with them. That is why they are calling you.

If they dont continue to give you internship pay then can go back to them.