(define (make-withdraw initial-amount)
  (let ((balance initial-amount))
    (lambda (amount)
      (if (>= balance amount)
          (begin (set! balance (- balance amount))
                 balance)
          "Insufficient funds"))))

; Is equivalent to
(define (make-withdraw initial-amount)
  ((lambda (balance)
     (lambda (amount)
       (if (>= balance amount)
           (begin (set! balance (- balance amount))
                  balance)
           "Insufficient funds"))) initial-amount))

; After (define W1 (make-withdraw 100))
 global env
------------------
|                |<--- env: global env
|                |     parameters: initial-amount
| make-withdraw: ----> body:
|                |       ((lambda (balance)
|                |          (lambda (amount)
|                |            (if (>= balance amount)
|                |                (begin (set! balance (- balance amount))
|                |                       balance)
|                |                "Insufficient funds"))) initial-amount)
|                |
|                |       E1
|                |     -----------------------
|                |<----| initial-amount: 100 |
|                |     -----------------------
|                |          /\
|                |       E2 |
|                |     ----------------
|                |     | balance: 100 |
|                |     ----------------
|                |          /\
|                |          |
|                |     env: E2
|                |     parameters: amount
| W1: ---------------> body:
|                |       (if (>= balance amount)
|                |           (begin (set! balance (- balance amount))
|                |                  balance)
|                |           "Insufficient funds")
------------------

; After (W1 50)
 global env
------------------
|                |<--- env: global env
|                |     parameters: initial-amount
| make-withdraw: ----> body:
|                |       ((lambda (balance)
|                |          (lambda (amount)
|                |            (if (>= balance amount)
|                |                (begin (set! balance (- balance amount))
|                |                       balance)
|                |                "Insufficient funds"))) initial-amount)
|                |
|                |       E1
|                |     -----------------------
|                |<----| initial-amount: 100 |
|                |     -----------------------
|                |          /\
|                |       E2 |
|                |     ---------------
|                |     | balance: 50 |
|                |     ---------------
|                |          /\
|                |          |
|                |     env: E2
|                |     parameters: amount
| W1: ---------------> body:
|                |       (if (>= balance amount)
|                |           (begin (set! balance (- balance amount))
|                |                  balance)
|                |           "Insufficient funds")
------------------

; After (define W2 (make-withdraw 100))
 global env
------------------
|                |<--- env: global env
|                |     parameters: initial-amount
| make-withdraw: ----> body:
|                |       ((lambda (balance)
|                |          (lambda (amount)
|                |            (if (>= balance amount)
|                |                (begin (set! balance (- balance amount))
|                |                       balance)
|                |                "Insufficient funds"))) initial-amount)
|                |
|                |       E1
|                |     -----------------------
|                |<----| initial-amount: 100 |
|                |     -----------------------
|                |          /\
|                |       E2 |
|                |     ---------------
|                |     | balance: 50 |
|                |     ---------------
|                |          /\
|                |          |
|                |     env: E2
|                |     parameters: amount
| W1: ---------------> body:
|                |       (if (>= balance amount)
|                |           (begin (set! balance (- balance amount))
|                |                  balance)
|                |           "Insufficient funds")
|                |       E3
|                |     -----------------------
|                |<----| initial-amount: 100 |
|                |     -----------------------
|                |          /\
|                |       E4 |
|                |     ----------------
|                |     | balance: 100 |
|                |     ----------------
|                |          /\
|                |          |
|                |     env: E2
|                |     parameters: amount
| W2: ---------------> body:
|                |       (if (>= balance amount)
|                |           (begin (set! balance (- balance amount))
|                |                  balance)
|                |           "Insufficient funds")
------------------

The difference in environments, is the implementation using let/lambda create
another environment storing the initial balance.