There is some indication that this event never actually happened, at least the papal approval. Rosamond McKitterick (who is a titan in the field) has a paper titled "The Illusion of Power in the Carolingian Annals" that talks about how this event was likely invented in order to create legitimacy for what was at its core a coup of the Merovingians. In addition the main papal source, the Liber Pontificalis (which contains the details of the lives of the popes) does not mention the event ever taking place. However, the real event takes place in 754 when Pepin is anointed by Pope Stephen. Here is probably where your question is more applicable. There is pretty good reason to believe that the pope and Pepin worked out some kind of agreement that was later encapsulated in the so-called "Quierzy document" which according to Thomas Noble was essentially the contingency plan if the Franks conquered Italy. If that happened the land listed in the document would be handed over to the papacy. In addition Pepin swore to uphold the "rights of St. Peter" from primarily Lombard incursion. In 755/6 Pepin would then lead an army to Italy in order to fight the Lombards.

Hopefully this is useful to you!

Sources: Royal Frankish Annals (references the supposed events of 751) Chronicle of Fredegar continuations (also references the event) Liber Pontificalis (main papal source that does not mention event) Rosamond McKitterick, "The Illusion of Power in the Royal Frankish Annals" Thomas Noble, The Republic of St. Peter