Using the Cr53 NMR, the field dependence of the magnetization M(T0,H) at elevated temperatures of single-crystal ferromagnetic CrBr3 has been determined. While an accurate fit to the M(T0,H)-vs-H data can be obtained using a two-exchange-parameter first-order renormalized spin-wave theory, the parameters required differ appreciably from those required for an accurate fit to the previously obtained M(T,0)-vs-T data with the same theory. A t-matrix two-parameter-model theory, correct to lowest order in the magnon density, but to all orders in the magnon-magnon interaction, was constructed. Although both the first-order and t-matrix corrections to the spin-wave energies are sizable, the full t-matrix results only change the large first-order corrections to M(T,H) by 15%. However, even if one employs the full t-matrix renormalization there are no pairs of values of the two exchange parameters which simultaneously fit the M(T,0)-vs-T and M(T0,H)-vs-H data. We attribute the inability of the more sophisticated theory to provide agreement with measurements of more than one thermodynamic function to be an inadequacy of the two-parameter model rather than an intrinsic failure of the theoretical approach.