mmmmmm, why would China GIVE an aircraft to Tonga? Interesting.
A lot of things are given to the pacific islands under the guise of aid, in return certain business favours are earned. Australia and NZ have been at it for years, as well as many other countries. Part of the reason Fiji has been in the naughty corner is that it is rejecting Aus/NZ involvement in favour of Chinese business. The trade agreements probably are similar to the Japanese situation where economic aid is given in the form of vehicles for police, naval vessels for patrol, aircraft etc... in exchange for favourable investment in local tourism businesses.