Maybe if they are stupid enough to send you the $6M without a signed release they'll be stupid enough to pay the mechanic without one too, but I have to think that a judge will probably look askance at your accepting the $$ and suing for more without an explicit agreement from the insurance company allowing you to preserve those overage claims.

You might also check the NY General Obligations Law and Insurance Law to see whether acceptance of a check for a claim constitutes satisfaction -- you'd obviously want to avoid sanctions for bringing an impermissible claim or having to defend an ins co counterclaim for fraud in taking the $$ and then turning around and suing their insured for a claim they've settled with you in good faith.

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'98 C43 AMG
'88 420SEL
'93 190E Limited Edition (including factory Sportline package)