Google Assistant is gaining a send money feature in the near future. Google recently debuted the new function at a session during their annual Google I/O developer conference, and just like with any other function that Google Assistant is capable of completing for you sending money is a rather simple task. You simply have to speak the trigger words to activate Google Assistant and tell it to send a specific amount to a person in your contacts list. It will be connected to your Google account, naturally, so whoever happens to be one of your Google contacts will be an available person to send money to.
You can see the action for yourself in the video below right around the 14-minute mark, and although it does show them asking Google Assistant to send "$10" to another user, presumably if you were to ask Google Assistant to just "send money" to someone it would ask you how much you want to send. After you get through that, it'll ask you to confirm the method of payment, i.e. which card you may want to pay with if you have multiple cards tied to your account, and then it confirms the payment has been sent and that' pretty much it.
There are still some unknown details about this feature, such as whether or not there is a limit Google Assistant will allow you to send, and of course when it will actually be available to those with devices that have Google Assistant baked in. The good news is that it will in fact allow you to use any card you have in your Google account so you won't be limited to a specific card, like a main card. You will have the choice to pick. While it's likely that it may be limited to your Google account cards at launch, Google is looking to add more p2p providers for payments through Google Assistant in the future so eventually you'll be able to select other payment methods, albeit without mentioning which those payment methods are at this time. One can assume though that it might be something like PayPal or something similar.