By Galina Sabeva SOFIA, Feb 25 (Reuters) - Bulgaria's indebted Balkan Airlines sealed a $4.49 million deal with British Airways (BAY) to swap 10 weekly runway slots at London's Heathrow airport for slots at London Gatwick, industry sources said on Monday. The cash compensation covers the difference in value of slots at Heathrow, Britain's busiest airport, and those at Gatwick and sheds light on the grey area of the monetary value of take-off and landing slots. "Under the deal Balkan will receive 3.15 million pounds from British Airways as a compensation for the slots' exchange," said a source who earlier on Monday attended a general meeting of the creditors of Balkan Airlines, put in receivership in March. The source said the administrators managing Balkan Airlines had informed the creditors about the terms of the deal, which was sealed last week, adding payment under the deal would be done in two tranches, the first of which was already complete. Balkan's new slots at Gatwick, London's second-busiest airport, had been valued at 600,000 pounds ($855,600), the source said. Balkan's court-appointed administrators had termed the deal with BA as "an exchange of slots with indefinite term". Another source, from Balkan Airlines, confirmed the price of the deal, adding part of the money would be used for leasing of two Boeing 737-300 aircraft from Germany's Lufthansa (LHAG). A Balkan spokesman declined comment, but said an official statement on the matter would be issued on Tuesday. British Airways on Monday declined to comment directly on the deal. "Any negotiations around slots we consider confidential so we won't be talking about any details," a BA spokeswoman told Reuters. "In general terms we are constantly talking to airlines for extra slots at Heathrow but we can't confirm any details." The deal puts a price tag on the value of runway slots, although it remains uncertain what rights airlines have to sell their slots at Heathrow and other busy airports in Europe. Last month BA rejected demands by U.S. regulators to give up 224 weekly slots at Heathrow, equivalent to 16 daily takeoff and landing times, to make more room for competition to its proposed alliance with American Airlines (AMR). A slot is a defined time period during which an airline can take off or land. SLOTS FOR MUCH NEEDED CASH Industry experts said the deal was part of BA's plans to reduce presence at Gatwick while seeking to increase the number of slots it owns at Heathrow. For the troubled Bulgarian flag carrier the slot swap is a last-ditch attempt to earn cash to cover operating costs. Aviation analysts said Balkan Airline's Heathrow slots were among its most valuable assets. The slots are scheduled for around midday, a convenient time for passengers from the continent to connect with transatlantic flights. Balkan operates six flights a week between Sofia and London, daily except Tuesday, requiring 12 slots per week. The airline will retain two slots at Heathrow to keep one flight a week. In mid-1999, Bulgaria sold 75 percent of Balkan Airlines to Israel's Zeevi Group but last February the airline was grounded for more than two months in a dispute over unpaid debts. Since May, Balkan has resumed flights to selected destinations, including London, Dubai and Tel Aviv, and later to Moscow, Paris, Rome, Istanbul and Copenhagen. From December it restarted flights to Zurich, Brussels and Helsinki. The airline's international fleet now consists of three Tupolev TU-154s, of which one is owned and two leased. Balkan's debts are estimated at 165 million levs ($74 million), of which 90 million levs are owned to the state. Other major creditors include oil refinery LUKOIL Neftochim (NEFT), Bulstrad insurer, Sofia Airport and global airlines body IATA. A local accounting firm has put its assets at 190 million levs.