If a person is salary, they only work 1 or two days is it the law that we have to pay them a full 40 hours?

There are too many variables here. Generally a person who is salaried is paid on a weekly or greater basis. If they didn't work a full week, why not? If they were sick, then what sick leave provisions does the company provide?

Please provide more enough so we can give you an educated answer.

We are a construction company we are slow right now. Some of the guys said they would work just for a day or two if we had the work. Someone told me that if a salaried employee works one or two days you have to pay them the entire week that it was the law. My question is if the employee agrees to it then is it OK to pay them for the days they worked and not the full 40 hours.

Most construction work is not salaried. If the employees agree to it, you can switch them to a per diem pay.

It's quite simple really.
If their contract states a 40 hour week and they work less, well they get paid less, its called pro rata (http://dictionary.reference.com/search?q=pro+rata)