An educational institute wanted to honour one of their supporters, and so they asked me to purchase a silver dish on their behalf for him, which they paid me for. Subsequently, they decided it wasn’t suitable and they asked me to return it to the store. I did so, and the store gave me in return a voucher for the amount of the dish. I took it hime, and after some time I realised that I must have misplaced it, because I can’t find it anywhere. Am I obliged to pay the institute the value of the voucher?
In general, unless you declared at the time that you took the voucher (or before then) that you accept no responsibility for looking after it, you would automatically become a Shomer Chinom, and would be responsible for negligence. Included in negligence is not knowing where you placed it. ((חושן משפט סי’ רצ”א סעי’ ז’ ))
Nonetheless, in this case you would not have to pay for the loss, because the voucher has the same status in this respect as documents that have no intrinsic worth other than the obligations stated within. Such documents are excluded from the Halachos of Shomrim ((שם סי’ ש”א סעי’ א’ )) , and as such you are not responsible for their loss. Although there are authorities that oblige payment even in such cases if the loss is caused through negligence ((שם ובש”ך ס”ק ג’ )) , the consensus amongst the Poskim ((ע’ רמ”א שם )) is to exonerate the Shomer of any financial obligations, especially if the plaintiff is not holding any money of the Shomer. As such, a Bais Din would not oblige you to pay for the loss even if it came about through your negligence.