To use a laundry machine in my yeshiva dormitory costs 14 NIS. I always pay for using it, sometimes before I use it and sometimes after (the money is put in a small bag; the machine is not coin-activated). For some reason, after using it recently, I simply couldn’t remember whether I payed or not before using it. Must I pay in this case of doubt?
If you used it and you aren’t sure if you paid, you night be stealing from the owner of the machine. Therefore you should pay him, and specify that if you are paying a second time, that you are being mochel the money. Alternatively if you can speak to the owner of the machine and he will be mochel the money to you, that is also fine.