Is a school allowed to withhold my son's transfer certificate over unpaid fees?

You are broadly right, and the school is on weaker ground than it claims. Education regulators in the UAE treat unpaid fees as a civil debt for the school to recover through proper channels — not as a licence to block a child's education by holding essential documents. In Dubai, the Knowledge and Human Development Authority (KHDA) oversees private schools and takes complaints about withheld transfer certificates; in Abu Dhabi that role sits with ADEK, and elsewhere with the federal education authorities. Your moves, in order: put the fee dispute in writing to the school, stating exactly which term is disputed and why, and pay any portion you do not dispute so the file shows good faith. Demand the transfer certificate in the same letter, noting the new school's enrolment deadline. If the school still refuses, file a complaint with the regulator with the correspondence attached — regulators have intervened in document-withholding cases, and schools tend to move quickly once one is involved. Keep the new school informed in writing so the delay is documented as the school's doing, not yours. The school remains free to pursue the disputed term through the courts, which is where that argument belongs. A UAE lawyer can also put the fee disagreement itself on a proper footing if the school escalates.