Hilchos Shemittah v’Yoveil 5:23

Straw and hay of shemittah may not be stuffed in a pillow or put in mud (e.g., for construction purposes). If one did put them in these places, they are considered to have been destroyed. If an oven was heated with shemittah straw and hay, it must be left to cool off. When the second rains of the eighth year arrive (i.e., in the autumn), we are permitted to benefit from shemittah straw and hay, and to use it as fuel.

Hilchos Shemittah v’Yoveil 6:1

One may not conduct business using shemittah produce. If a person wants to sell a small amount of shemittah produce, he may do so. The money he receives from such a transaction has the same status as the shemittah produce: he must use it to buy food, which he must eat according to the rules of shemittah sanctity. The produce he sold retains its sanctity.