A problem with this scheme would be a DOS vulnerability. If the consumer negotiates the deal but denies or delays the signature, the producer would either have to build on the prior level of negotiations between the middle-man and him with the next customer (risky-ish), wait for the signature with a timeout (blocking. that's a no-go in almost all scenarios) or negotiate a new channel for as much parallelism he needs (costly-ish. has to be implemented for client and middle-man and results in multiple channels between one provider and one middle-man). The good news is that protecting against this type of DOS-attack would justify to charge a higher premium for the middle man.