The legal argument that could be made was that Hussein as sovereign king could expropriate land just as governments in the US do, under eminent domain. But do make that argument, you’d have to concede the legitimacy of his occupation of Juda and Samaria after 1948, and annexation after 1953. The Palis, then, in order to claim ownership of the land by virtue of a grant from King H, would have to eschew their own right to it as a sovereign nation (that never existed). Perhaps they could claim that his acts as sovereign were legitimate during his time of occupation, but then they’d open the door and have to explain why Israeli sovereign acts are not legitimate during a similar period. No matter how you look at it, they cannot make a logically consistent claim to the land based on those grants.
The only problem is that he was giving away land to compensate so called Palestinians for murdering them in Jordan. That was taking place after Israel had already taken Judea and Samaria in 67, so there is no standard by which that was Hussein’s land to give away.