The Hollywood star split from Grigorieva, the mother of his daughter Lucia, early last year (10) and she went on to accuse the actor of beating her in a violent domestic dispute.
Her teenage son Alexander Dalton, from her relationship with former James Bond star Timothy Dalton, was reportedly present during the altercation, although he is not thought to have actually witnessed the bust-up.
Grigorieva subsequently dropped her allegations of domestic violence against Gibson and they settled their bitter custody battle over young Lucia last month (Aug11).
In that agreement, Gibson was ordered to pay his ex a total of $750,000 (£468,750) in three equal installments over the next five years.
However, legal documents filed by Grigorieva's representative allege the actor offered the extra cash to Alexander Dalton if he waived his right to sue over the bust-up.
The court papers read: "If Alexander Dalton does not get his $100,000.00, he cannot waive his rights to sue. Mr. Gibson negotiated for that as a part of the settlement and is entitled to the benefit of his bargain."
But Gibson's spokesman has contradicted the claims, insisting, "Mel is paying exactly the sums set forth to (Grigorieva) - and is paying no additional sums to Alexander Dalton or anyone else. How OG (Grigorieva) chooses to have any sums due from Mel allocated is her business."