Not sure what you’re talking about. I’m not American, but this action is pretty clear to me. Milei came into power promising to resolve the economic crisis of Argentina through extreme liberalisation of economy and with stronger bonds with the USA. The USA indeed benefits from these bonds, through the privatisation of companies and natural resources as well as the possibility to act as a foreign investor. However, the economic crisis has not been resolved: life is expensive and salaries are low. This year there are elections in Argentina, there is the possibility of a change of government to a party which may not be so friendly to the USA. By giving Milei a large sum of money Trump gets more support from Milei directly (and probably some favours and concessions) as well as a higher chance that Milei gets reelected. With that money Milei can give subsidies and enact reforms which the people will appreciate and point out how his reforms worked out, as well as show that his bonds with the USA are beneficial to the country.

You say Trump is draining the economy of USA for his personal gain, but this very much does not seem the case. Looking at the history of Latin America this looks much more as a small investment to drain the Argentinian economy.