If playing with the 10 point penalty, then the penalty would be incurred in this case.
First of all, the 10 point penalty is not a universal rule. Out of many different sources of Canasta rules, I only found two that mention a 10 point penalty for an illegal meld.
If you put down an initial meld that doesn’t meet the minimum point requirement, you can put down additional melds on that same turn to make up the required total without penalty. You can also take back your attempt, but the minimum total you have to meet the next time you try goes up by 10 points — quite a severe penalty.
(https://www.dummies.com/games/card-games/canasta/playing-canasta-making-the-first-meld-for-your-partnership/)
And
If a player melds fewer cards than the minimum count for his first meld, he may either add additional cards to the meld(s) until they reach the minimum count, or he may withdraw them, in which the minimum count increases by 10 points. If the opponents do not catch the error until after they've drawn, then the initial meld stands.
(http://www.rummy-games.com/rules/canasta.html)
Although the wording on the first one is not completely clear; the second one is clearer and sounds like it is meaning to say the same thing: That the only way to avoid the penalty is if you can just add to your illegal meld to make it legal. If you have to actually undo your meld, then you take the penalty.
For reference, here are 3 other websites with Canasta rules that do not mention any 10 point penalty:
