Strategy Page
In October 2017, after a year of negotiations over terms, Russia began delivering free military equipment to Serbia. The most visible items were the six MiG-29 fighters that were supposed to arrive in March 2017 but delays were experienced. There was no explanation for the delays but is was believed to be all about the terms of the gift (which includes 60 armored vehicles and various other items). This deal was meant to save Serbia a lot of money but the Russians wanted the Serbs to pay for upgrades to bring these MiG-29s up to the latest standards.
https://www.strategypage.com/htmw/htmurph/articles/20171101.aspx
