Sanding and refinishing cost

I have this stain on my floor. Its because my bed was broken and i slept on the floor for few days and there was a paper below it which got wet. Although, i put plastic below the mattress to prevent the wet mattress contact my floor but the paper on the remaining place was a bad idea. I am looking for ideas how i can get it fixed? What i need to do? How much would be cost? I need to do it before leaving the apartment