Hi Deepika
That is the default behavior of the Jettison library used to perform JSON conversion in the REST adapter. For more details, read Vadim's blog below for an in-depth explanation of it.
REST Adapter in PI/PO: Enhanced XML/JSON Conversion
The blog also covers the functionality on how to enhance the default conversion, i.e. for your case, configuring the particular field to be converted as a String instead of decimal. You'd need to ensure that you have the correct SP level for your system to have that functionality.
Regards
Eng Swee