Is the father contesting paternity? Just because an Affidavit of Paternity was signed by the father doesn't make it absolute - it is an assumption. What a person thinks at the time of birth may change later, for example if your child appears to be the product of a mixed race couple and the both of you are of the same race. Some states limit the amount of time to challenge paternity and state law may vary regarding who pays. In this instance, it may be the father who pays as he may be challenging paternity after signing the affidavit. I'm not 100% sure. Here is also a relevant statute that might be of assistance.
Alabama Code, Section 26-17A-1: Reopening paternity case.
(a) Upon petition of the defendant in a paternity proceeding where the defendant has been declared the legal father, the case shall be reopened if there is scientific evidence presented by the defendant that he is not the father. The court shall admit into evidence any scientific test recognized by the court that has been conducted in accordance with established scientific principles or the court may order a blood test, or a Deoxyribose Nucleic Acid test of the mother, father, and child. Whenever the court orders a test and any of the persons to be tested refuse to submit to the test, the fact shall be disclosed at the trial, unless good cause is shown.
(b) The test shall be made by a qualified expert approved by the court. The expert may be called by the court or any party as a witness to testify to the test results and shall be subject to cross-examination by the parties. The test results may be admitted into evidence. If more than one test is performed and the results are conflicting, none of the test results shall be admissible as evidence of paternity or nonpaternity.
(c) Compensation of the expert witness shall be paid by the petitioner.
(d) In the event the child has been adopted the matter of paternity may not be reopened under this chapter.
