First, all parties have secure elements so they can probabilistically make payments. The honest and optimum payment promise is:
No arbitrage cash does a lot of heavy lifting behind the scenes, especially with optimum congestion. If the contract is valid at the decision point, then both the distribution of arrivals for the part and the service are known, odd of 'fails to deliver' can be computed, a priori. Of course, the system is not stationary, that is, a payment on the 15th may hold for a while, but there is nothing magic about 15 in the sandbox. All parties in the contract strive to out perform delivery on the margin. So there is a strong element of uncomputable risk, but currency risk is handled properly.