A boy my stepson used to be very good friends with stole his YuGiOh deck and started selling it. Once we found out through the grapevine that J had SS's deck, DH got involved and confronted the kid and his mom. The kid/mom paid to replace any cards (all the good ones, of course) that J had already sold. It was not a good situation, but SS needed us to intervene. J is a troubled kid and SS says that now he's a waste of space. For a bit after the incident, SS stayed friends and we set up very clear boundaries. SS took NOTHING over to J's house. J wasn't allowed here to hang out. We put down these rules because we had to look out for the rest of the family, too. The friendship did not last long after that, but it came to an end on SS's terms.
We've also had incidents where a game was taken to a party by SD and the game went missing and nobody wanted to accuse another guest so it was just gone. SD similarly learned not to take things like that with her.
BUT there were also times when money or items just got lost by our kid. SD would clean her room and unearth whatever she thought was gone. We had to have discussions with her on the difference between "stolen" and "lost". She would often say something was "stolen" and later admit it was left behind by herself. Sigh. Or the time that SS took his video games to a match at another school and left them in a bag on the bench in the locker room. Um, sorry, but that was his own fault for not locking them up. And he lost his wallet with money and ID in it when he was a new driver...we made him pay to replace the ID.
So...this is a lesson for your son about his money. The money is gone. Either it was lost by his poor placement or stolen, but since you have no proof, then you can only deal with what you know - that DS left it on the floor. If you think that this child cannot be trusted, change the rules for future playdates and see how it goes. You can also consider asking again if he kid saw it or talking to the parents yourself (as your son is only 8). But at the end of the day, if it's not to be found, then it's something your son will have to learn to deal with, upsetting though it be.