What is the ruling of Islamic law regarding the money of a deceased wife which her parents relinquished to their minor daughter, and is the husband entitled to dispose of it as her legal guardian, knowing that they had previously absolved him of it?

Your wife's parents waiving their share of their daughter's inheritance in favor of their granddaughter (your daughter) is valid if they are legally competent to do so. In this case, your daughter will own their share in addition to her original share. The hadith "You and your wealth belong to your father" means that it is permissible for a father to consume from his son's wealth and dispose of it when needed, in a reasonable manner. It does not mean that the father owns his son's wealth; the wealth remains the exclusive property of the son. Therefore, based on this hadith, it is permissible for you to manage your daughter's wealth and consume from it when needed.