In my yeshiva dormitory, the wash machine costs 7 nis to operate and so does the dryer. The yeshiva’s policy though is that yeshiva property – specifically, towels and floor rags – can be washed and dried free of charge.
For weeks, 10-12 of the Yeshiva’s rags were dirty and unwashed. I happened to have 4 dirty rags of my own and wanted them clean. So I washed the yeshiva rags together with my own in a wash cycle and then in the drier. Do I need to pay anything for this?
We can assume that the yeshiva isn’t going to mind, as long as you are not hurting the was or costing them anything extra, i.e detergent. Then we would say that it is זה נהנה וזה לא חסר.