Is it permissible for someone who is unsure about the exact amount of zakat due on his wealth to pay a larger sum with the intention that part of it covers the obligatory zakat and the excess is considered a voluntary charity, or is it better to pay the entire amount with the intention of it all being zakat?

One who doubts the amount of Zakat obligatory upon him is not required to pay إلا ما تحقق وجوبه (except what is confirmed as obligatory), because the default is the exoneration of one's liability. As for whoever wishes to pay more than what is obligatory, let him intend the extra distinguishable amount as a voluntary charity (Sadaqat al-Tatawwu’). However, concerning an indistinguishable increase, such as paying an older animal as Zakat for livestock, there is a difference of opinion as to whether all of it is obligatory or part of it is voluntary. And whoever doubts whether the wealth reaches the minimum threshold (nisab) or not, Zakat is not obligatory upon him, though it is preferable to be cautious and pay it.