Is one considered sinful if he retracts from donating a Mus'haf to the mosque and instead reads from it in his home?

If your statement, "I will give you a Mushaf for the mosque," is merely a promise, then fulfilling it is commendable (mustahabb). However, if you intended that you have actually endowed this Mushaf to the mosque, then you are obligated to fulfill it, because an endowment becomes binding through intimation (kinayah) if it is accompanied by the intention of endowment, by agreement of the imams. Therefore, sin would accrue to you if you withheld what you have endowed from its rightful beneficiaries, and you must fulfill it immediately if what you uttered was accompanied by the intention of an immediately effective endowment (waqf munjaz).