Is it considered against Islamic law not to buy replacement gold for a divorced wife, given that the gold was sold after the marriage and was not listed in the inventory of movables, and it was a gift to the husband, and the husband agreed to return the deferred mahr and all other listed movables?

The "shabka" (engagement gift) is part of the Mahr (dowry). If the husband sells it without his wife's consent, the sale is invalid, and he must return it, or an equivalent item, or its value at the time of restitution. The same applies if he took it as a loan. However, if the wife willingly gifted it to him, then she has no right to it.