Hi Vikas,
Before posting blindly on SCN, did you try testing it on your system.. Did you try posting invoice with reference to 105 document.. please some effort to test it.. you may able answer yourself..
GR-IR indicator enable you to use the Delivery note captured in the material document. Delivery note always take reference from 103 document.. You can try .. if you put delivery note number in 105 document but not in 103 document.. system does not allow you to enter delivery note number..