Hi Gopi,
At this Wiki, you can find some of the restrictions of ABAP OO. This is quite interesting literature :
http://wiki.scn.sap.com/wiki/display/ABAP/ECC5.0+And+Above
Concerning your FM : if you call the FM from the method and you debug it, does the conversion work in the FM if you debug it? If it does, what is the type of the parameter that you return the value in? If the conversion works in the function and you return it to a decimal in the method, I do not see where it goes wrong.
If you show the code of the method (with parameters) and the function, I can test it myself.
Regards,
Freek