Understanding Frustration - Contractual Discharge by Agreement

Mujhe abhi tak lagta tha ki koi bhi contract discharge ho sakta hai, lekin ab samajh gaya hai koi agreement toh aisa hi hota hai. Ek example dekhkar: Niranjan Das v/s Govt. of West Bengal (1958) - main contract discharge ke liye, parties ne express agreement sign kiya. Lekin, yeh hi baat pata chali ki agreement mein koi specific conditions hote hain, jis par agreement ka rely karna hota hai. Toh ab hum kya seekhte hain? Agreement wale conditions ka puraana, existing rights ko accept karna hota hai. Aur agar koi specific performance ke case me hoga, to wahi hota hai.