Relying on information published on another web site, the amount and statutory justification for the merchant returned check fee in South Carolinais: $30 and S. C. Code Ann. f 34-11-70 (2002). (I do not have access to S.C. statutes, so I cannot take it from there.) I doubt that it requires you to sign an acknowledgement. As noted, a sign is probably all that is required. Your signature on the back of the check would be "surplusage" to the drawee bank; i.e. it would be unnecessary and ignored if properly endorsed by the payee.
It's the depositary bank that has the most to lose. If the restaurant's homemade remedy obscures the depositary bank's endorsement and causes the check to be late in being returned, they assume the risk.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.