Russia is suspending its program to upgrade Syria's MiG-31 fighters. In 2007, Russia and Syria signed an agreement by which Russia would provide the country with seven MiG-31 aircraft, as well as equipment and services to upgrade Syria's aging fleet of combat aircraft. The total value of the deal was estimated at $400-500 million. The problem, as usual, revolves around money. Syria simply does not have the cash to pay for the program. This is despite the fact that Russia forgave 70 percent of Syria's debt to them back in 2005. The Syrians had owed over $13 billion.
Syria has long been indebted to the Russians for years, much of it coming from arms and equipment purchased from the Soviet Union during the Cold War. During 1970s and '80s, Syria received large amounts of sophisticated weaponry from the Russians on excellent financial terms. This was due to the fact that the Soviets were more interested in maintaining political influence in the region and counterbalancing American influence. Obtaining and keeping allies was paramount. These days, making money is the number one priority for the Russians and the country needs customers who can actually pay. Also, Russia's interest in maintaining their sphere of influence is far less global than during the Cold War, with the Russian Federation almost entirely concerned with maintaining hegemony over the countries that border it. Preoccupied with their own defense reforms and the need to raise more cash quickly, the Russians simply can't afford to be as generous with giving away equipment as they used to be.
Also, the enormous expense involved in upgrading and obtaining even seven new combat aircraft would potentially take a crippling chunk out of Syria's already pitiful military budget. Syria can only afford to spend around a billion dollars, sometimes less, on its military annually and the since the deal demanded that the country pay out at least $400 million for the new planes and upgrades, this would potentially cut into the budget so much that it could even endanger the Syrian' ability to maintain any kind of forces at all, even if the money were paid out over a long period of time. The Syrians want to rebuild their armed forces back to the state they were during the 1973 and '82 wars with Israel, when they were largely equipped with up to date weapons and managed to maintain some professionalism. As it stands, the Syrians can barely afford to keep the equipment they do have running, much less pay out of pocket for new gear. The Syrians simply don't have the money and the Russians are less generous than in the past.