1. Put bison sirloin in a resealable bag. Combine wine, olive oil, garlic, oregano, salt and pepper, and pour mixture over bison. Refrigerate and marinate at least 4 hours, preferably overnight.
2. Heat a skillet over medium to medium-high heat. Melt butter and add bison mixture. Cook, stirring occasionally, until sauce is mostly evaporated, about 10-15 minutes.
3. Meanwhile, combine yogurt, cucumber, salt and pepper.
4. Toast pita breads in a hot pan, about 30 seconds on each side.
5. Top pitas with bison, lettuce, tomato, and yogurt sauce.