Jelly, 8 7 bytes

Vf96SạP

Input is as an array of two integers: first the right opererand, then the left one.

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How it works

Vf96SạP  Main link. Argument: [b, a]

V        Cast [b, a] to string, then eval the resulting string.
         For [b, a] = [9, 6], this yields 96.
 f96     Filter with 96, yielding [96] if V returned, [] otherwise.
    S    Take the sum, yielding either 96 or 0.
      P  Compute the product of [b, a], yielding ba = ab.
     ạ   Compute the absolute difference of the results to both sides.
         When the sum is 0, this simply yields the product.
         However, when [b, a], = [9, 6], this yields 96 - 54 = 42.

Jelly, 8 7 bytes

Vf96SạP

Input is as an array of two integers: first the right operand, then the left one.

Try it online!

How it works

Vf96SạP  Main link. Argument: [b, a]

V        Cast [b, a] to string, then eval the resulting string.
         For [b, a] = [9, 6], this yields 96.
 f96     Filter with 96, yielding [96] if V returned 96, [] otherwise.
    S    Take the sum, yielding either 96 or 0.
      P  Compute the product of [b, a], yielding ba = ab.
     ạ   Compute the absolute difference of the results to both sides.
         When the sum is 0, this simply yields the product.
         However, when [b, a] = [9, 6], this yields 96 - 54 = 42.

Jelly, 8 bytes

42P⁼?6,9

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How it works

42P⁼?6,9  Main link. Argument: [a, b]

    ?     If...
   ⁼ 6,9    [a, b] == [6, 9]:
42            Yield 42.
          Else:
  P           Call product, yielding ab.

Jelly, 8 7 bytes

Vf96SạP

Input is as an array of two integers: first the right opererand, then the left one.

Try it online!

How it works

Vf96SạP  Main link. Argument: [b, a]

V        Cast [b, a] to string, then eval the resulting string.
         For [b, a] = [9, 6], this yields 96.
 f96     Filter with 96, yielding [96] if V returned, [] otherwise.
    S    Take the sum, yielding either 96 or 0.
      P  Compute the product of [b, a], yielding ba = ab.
     ạ   Compute the absolute difference of the results to both sides.
         When the sum is 0, this simply yields the product.
         However, when [b, a], = [9, 6], this yields 96 - 54 = 42.

Jelly, 8 bytes

42P⁼?6,9

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Jelly, 8 bytes

42P⁼?6,9

Try it online!

How it works

42P⁼?6,9  Main link. Argument: [a, b]

    ?     If...
   ⁼ 6,9    [a, b] == [6, 9]:
42            Yield 42.
          Else:
  P           Call product, yielding ab.