The Bills tried to sign Nixon from the PS, but the Packers did not want to lose the speedster. The Bills are famous for trying to sweep players off the practice squads from other teams. This was a failed attempt though.
If you remember when Chan Gailey was the head coach in Buffalo, the Bills tried to steal Graham Harrell from the Packers practice squad, but the Packers were given an option, and re-signed him as well.
The Packers had to make a move to keep Nixon, and they placed Francois on the I/R, ending his season. It was a gamble but the Bills came up short.