Low-field harmonic susceptibilities of a Ni77Mn23 alloy have been measured in the temperature range 15-325 K at three different frequencies. The measurements exhibit both frequency and amplitude dependencies. Observed dependencies are compared with the existing theories of linear and nonlinear susceptibilities with reference to short- and long-range interactions. Although the sample exhibits some features similar to the ordinary spin-glass systems at low temperatures, the Curie temperature T-C was found to he larger than expected. However, the sample exhibits some interesting properties in terms of spin frustration below T-f at some frequency and amplitude. The coexistence of spin-glass with ferromagnetic states at low temperatures signifies the nondivergent behaviour of the high-order harmonic susceptibilities in the vicinity of T-f. The critical exponents gamma = 3.2 +/- 0.2 and beta = 1.68 +/- 0.4 suggests that the third harmonic susceptibility is essentially different from the observed divergent behaviour in spin glasses.