When my daughter was in primary school, I bought her a winter coat - she wore it to school, hung it in the cloakroom, another girl thought the coat was hers and wore it home, herself. My daughter picked up an identical coat, thinking it was hers, and brought that one home. Except this one was too big, at least one size, possibly even two sizes too big, for my daughter. The next day, I went into the school and said "accidental swapping of two identical coats - can you get the girl with my daughter's sized coat to swap back, please?"
Well, in theory that would have worked - but when the school alerted the other mother, she said "oh, I thought my child had outgrown her new coat, because it was too small, so I took it to a charity shop on the way home and bought her a new one!" She hadn't thought to ask her child if there was another identical coat hanging in the cloakroom (my daughter had said that there was, when asked), and had just presumed that her daughter had outgrown a brand new coat within a few days of having it... and given it away.
I remember being quite cross about the fact that the other mother had given my daughter's coat away, but there was nothing I could do about it. So my daughter wore a coat that was too large for her that winter...
Keep the shoes. Even if the other boot can be found, you have no idea what state it's likely to be in (things left lying around school cloakrooms tend to end up kicked around by the kids, in my experience). If the other parent is that bothered... he'll just have to buy his child a new pair himself, won't he? Why should your daughter end up with just one boot, and you out of pocket because it took a month for his child to admit to losing one boot, "oh, and by the way, Dad, they've mysteriously grown a size bigger!"... 