* Deal set to complete on Dec. 18 * 5 bln stg to be returned via special dividend * Pension fund to get 2.5 bln stg injection * Tesco shares up 1.3% (Adds detail, analyst comment, updates shares) LONDON, Dec 9 (Reuters) - Shareholders in Tesco can look forward to a 5 billion pound ($6.7 billion) windfall in the new year after the group said it would complete a $10.6 billion sale of its Asian businesses to Thailand’s CP Group next week. Tesco agreed to sell its businesses in Thailand and Malaysia to Dhanin Chearavanont’s CP Group in March. Britain’s biggest retailer said that CP Group, which runs its 12,000 7-Eleven convenience stores through CP All and about 80 cash-and-carry stores under Siam Makro, was satisfied with the approval notice from Thailand’s Office of Trade Competition Commission.