Is the questioner burdened by sin for not ensuring the disappearance of the letters on a burnt Quranic paper buried in soil that might contain sheep droppings?

There is no blame upon you for what you did, burning and burying the paper containing Quran, rather you are hoped to be rewarded, because your intention was to preserve the Quran. Burning alone is sufficient to dispose of damaged Mushaf (definition) pages, so what about when burial is added to it? The Permanent Committee for Issuing Fatwas has permitted burning or burying worn-out Mushaf pages to preserve them, just as the Companions did. As for the dung mentioned, it holds no significance after it has mixed with the earth and a long time has passed.