Not actually true. A lot of Russian arms sales, especially for more advanced stuff beyond guns and simple rockets (e.g. s-300, vehicles and craft) come with a written contract not to transfer to another party without authorization from Russia.
I know this because Russia was howling about some Latin American country recently (Ecuador) which had a bunch of Russian equipment considering sending it to the US where it would then be sent on to Ukraine in exchange for some US equipment and Russia was claiming it was a violation but of course they were pretty powerless to stop it. The most Russia can do in most cases of violation is simply refuse to sell to those people again but if you're already taking US arms and stuff and maybe siding with the evil empire then that doesn't matter much to you or your growing bank account with CIA deposits as a leader.
Link: https://www.rt.com/news/592712-ecuador-reverses-ukraine-arms/
Moscow had slammed the plan, saying that it would be a violation of contracts to transfer military equipment to a third party without Russia’s consent. Russian Foreign Ministry spokeswoman Maria Zakharova told Ecuadorian media that Quito had made “a rash decision” under outside pressure.
Global south buys from Russia because Russia and formerly the Soviet Union has been their friend while the US and Europe their oppressors. Russia also offers better value.