Yes. opennds.
Consider though, that a voucher system is very much a thing of the past, because of the ease of availability of 4G data. A public "hotspot" nowadays is something that is desired to attract customers to the venue and must by definition be free (albeit with authentication and basic logging to comply with legal requirements).
Having said this, a voucher system does still have its place in countries where Internet access is still not yet generally available.
OpenNDS supports development of just about any kind of portal using its API, and a fully functional voucher system running entirely on the router has been developed by the community: