fail.go

package main

import (
	"encoding/json"
	"fmt"
)

type Sale struct {
	Description string
}

func (s *Sale) ToMap() map[string]interface{} {
	return map[string]interface{}{
		"description": s.Description,
	}
}

func (s *Sale) MarshalJSON() ([]byte, error) {
	return json.Marshal(s.ToMap())
}

type Payment struct {
	ID   int
	Sale *Sale
}

func (p *Payment) ToMap() map[string]interface{} {
	return map[string]interface{}{
		"id":             p.ID,
		"sale":           Map(p.Sale),
	}
}
func (p *Payment) MarshalJSON() ([]byte, error) {
	return json.Marshal(p.ToMap())
}

type Mapper interface {
	ToMap() map[string]interface{}
}

func Map(m Mapper) (result map[string]interface{}) {
	if m == nil {
		return nil
	} else {
		return m.ToMap()
	}
}

func main() {
	var emptySale *Sale
	payment := &Payment{ID:2, Sale:emptySale}

	fmt.Println(payment.MarshalJSON())
}