if you find the voucher and you use it to play the slot machine I think its possible but its better to avoid the risk that will happen because usually every slot machine owner has their own rules so it would be better for you not to use it and it would be better for you to remember the purpose of your coming go there to have fun with the amount you have and give the voucher to the casino employee.

regarding the risk that occurs if you use the voucher, perhaps the original owner will look for it and ask for the voucher you used back and disturb you when you are playing the slot machine as told by @btc_angela. even If I have never had an experience like this, I can understand what it would be like if it happened to me because after all the voucher you found is not yours and the original owner still has the right to ask for it back.