Hi Anshuman,
I would assume that the '&' sign was implemented for a reason and wouldnt suggest to fix it in the Source system. Instead if you really don't need this in BW, there are two ways of solving the issue.
1. Write an Enhancement statement to just delete the unnecessary sign and update them back, so that the extraction from Source system itself would be Sign free.
Note : If you donot want to touch SAP standard column, you could enhance a column and then delete the unnecessary sign.
2. Extract the data as is into BW(i.e PSA) and then eliminate the unnecessary character in the start routine of your transformation.
Regards,
Thejas K