advice needed for FT recruitment

I am a student from a target school. I had an operation (back office) summer internship at a bank last year and I have secured an FT offer from the internship. However, I have just got an offer for another BB's front office role and the hiring manager asked if I have gotten an offer from the summer internship last year. I know reneging is frown upon but I am much more interested in the front office job. My plan initially was to take up the ops job and network my way to the front office, however, taking up a front office FT job straight will be much faster. How should I tell the hiring manager that I have the return offer? Will it affect my FT offer since it means reneging on the return offer? Any advice is appreciated!