Ferromagnetic exchange stiffness is a key to describe the robustness of a ferromagnetic interaction. Because the experimental determination of the exchange stiffness constant A continues to be elusive, especially at elevated temperatures, micromagnetic insights on the impact of thermal fluctuations on the ferromagnetic spin order have remained less tractable. In this study, the temperature dependence of A is experimentally determined for a Ni2Mn0.8In1.2 single crystal by measuring the 180 domain-wall width using off-axis electron holography and magnetocrystalline anisotropy. The theoretically expected power-law scaling with magnetization for A is shown to be applicable only in the low-temperature region (approximately half of the Curie temperature), and A starts to follow the power law with a significantly larger scaling exponent in the higher-temperature region. The decremental trends in the domain-wall width and the exchange length at elevated temperature seem to be attributed to this temperature dependence of A.