Hi Lokesh,
This is happening as the data element to with your property UOM is bound to, might be having a conversion exit in the domain. By default, Gateway enables conversion exit. It is possible to disable the conversion exit for the entire model and not for a specific property, in your model class as follows:
model->SET_NO_CONVERSION( iv_no_conversion = abap_true ).
If you are using the SEGW tool you can do the same in your model extension class.
Best regards,
Aakash