Once your law department has selected a firm though a competitive bidding process, there still remains the chore of executing an agreement with the firm. The good news, however, is that the agreement should not take long to finish because you have appended the form of it to the second-round of your RFP process (See my post of March 30, 2008: RFP process with 22 references.).
Request that the finalist firms review it and let you know if they have any major issues. If a firm takes issue with some part, they must explain that in their final bid and you can take the suggested revision into account when you evaluate the finalists.