Currently this feature has been made available in the UK on the beta build version of Signal. It is believed that Signal may soon release it in other countries as well. Signal payment option is visible on the beta version in the UK. With this, users will be able to easily send and receive 'privacy focus payment' messages.
The Mobilecoin Wallet feature is performing the Signal Payment feature under a protocol. Mobilecoin wallet has its own currency. It is called MOB. The company told in the blog that once you add Mobilecoin Wallet with Signal, you can send and receive money.