//load a sale by ID 
var sale = PublicAPI.CRM.Sale.Load(3);

//create an inpayment 
var myInpayment = PublicAPI.CRM.Sale.CreateInpayment(sale);

//myInpayment - created inpayment, linked to the sale 

//WARNING!! An inpayment is created without a name. For correct work with the inpayment 
//you should specify its name and save it to the database
myInpayment.Name = "Inpayment Name";
myInpayment.Save();