Is it probable for the impurity of feces to transfer from underwear to outer clothes due to sweat, given that the residue did not directly reach the outer garment?

The default is that ritual impurity (najasa) does not transfer and purity remains. So, whenever we doubt whether something has become impure, we assume the original state, which is purity, as certainty is not removed by mere doubt. Therefore, your outer clothing does not become impure merely by doubting the transfer of impurity to it through sweat. Consequently, you are not required to wash it; rather, its purity is affirmed.