To exchange money with other players, you must first create a cheque (or check) and give it to them. Once they have received it, they simply right-click with the cheque in their hand to cash-in the cheque

Cheque Demo

Creating Cheques

Use the /cheque <amount> to create a cheque with a specific amount stored

You must have some place available in your inventory to create a cheque

Cheque Create Demo