Did God obligate Himself to anything that would entail the requital of a servant, and is this still from God's mercy, given that God is the One who obligated it upon Himself?

It is not possible to definitively distinguish between the two statements in the questioner's mind, because he has reverted to the idea that Allah has made it obligatory upon Himself, and this does not contradict the first statement. The saying: "Nothing is obligatory upon Allah except what He has made obligatory upon Himself" does not imply compulsion, but rather a commitment from Allah Almighty out of His grace and mercy. This is His promise, which He does not break. The verses have stipulated that Allah "Your Lord has inscribed mercy upon Himself," and scholars have interpreted this to mean that He has made it obligatory upon Himself out of His favor and benevolence, an obligation of generosity, not an obligation of necessity. This is supported by the Hadith (definition) of the Prophet, peace and blessings be upon him: "When Allah decreed the creation, He wrote in His Book, which is with Him: 'Indeed, My mercy prevails over My wrath.'"