differential signal in cyclone 3

Select a differential pin pair and assign a differential I/O standard to the positive pin in the Pin Planner or Assignment Editor. The negative pin is assigned by Quartus automaticly.

why do you need a differential signal for such a slow signal?

just for example i explained ,...is there any problem?

Of course not, you can use a differential standard for a DC signals as well.

There is no problem, and everything should work, but usually one uses differential signals for high speed signals.