Gravitational wave (GW) measurements provide the most robust constraints of the mass of astrophysical black holes. Using state-of-the-art GW signal models and a unique parameter estimation technique, we infer the source parameters of the loudest marginal trigger, GW170502, found by LIGO from 2015 to 2017. If this trigger is assumed to be a binary black hole merger, we find it corresponds to a total mass in the source frame of $157^{+55}_{-41}~rm{M}_odot$ at redshift $z=1.37^{+0.93}_{-0.64}$. The primary and secondary black hole masses are constrained to $94^{+44}_{-28}~rm{M}_{odot}$ and $62^{+30}_{-25}~rm{M}_{odot}$ respectively, with 90% confidence. Across all signal models, we find $gtrsim 70%$ probability for the effective spin parameter $chi_mathrm{eff}>0.1$. Furthermore, we find that the inclusion of higher-order modes in the analysis narrows the confidence region for the primary black hole mass by 10%, however, the evidence for these modes in the data remains negligible. The techniques outlined in this study could lead to robust inference of the physical parameters for all intermediate-mass black hole binary candidates $(gtrsim100~mathrm{M}_odot)$ in the current GW network.
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