The effect of the Great Rings upon Men

[Inziladun]

Originally Posted by Zigûr

Perhaps Gandalf knew or assumed that, as a Dwarf, Thrór would be more resistant to some of the effects of the Ring, and thus more capable of passing it on to his son?
It could be argued that when Gandalf says to Frodo that no one ever gives up a Ring freely, he's specifically talking about Men (and, by extension, Hobbits).
I realise this is narrowing things down a lot but at the same time it does seem to suggest that he's giving Frodo the information which is most relevant to his particular situation.

Since it's clear that the Seven did not have the same effects on Dwarves as the Nine had on Men, I don't see any issue with the idea that the Dwarven Keepers were capable of freely passing on their rings.

That would not have been true of the One though. It was far more potent than any of the 19 others.