Depp's lawyers are trying to appeal the ruling in his U.K. defamation case, in which he sued The Sun for libel over a 2018 article labeling him a "wife beater" ... but he took the L in November when the judge ruled there was evidence he'd abused Amber.

Amber was The Sun's star witness and took the stand to lay out her allegations of Depp's domestic violence during their marriage.

But, Johnny's lawyer, Andrew Caldecott, claimed in court Thursday that Heard's testimony was unfair because her promise to donate the $7 mil divorce settlement provided a "considerable boost to her credibility as a person" and "tipped the scales against Mr. Depp from the very beginning."

Translation: Depp's lawyer thinks the judge believed Amber because of her donation claims, but she hasn't actually donated all the money. In fact, he says she's only donated $100k of it ... so he thinks that entitles his client to a re-trial.

Here's the thing, though ... Heard's attorney responded to these claims already earlier this year. The lawyer, Elaine Bredehoft, admitted Amber has not yet fulfilled the entirety of the $7 mil pledge she made to the Children's Hospital Los Angeles and the ACLU ... but says that's only because Depp keeps suing her.

Bredehoft said, "Amber has already been responsible for seven figures in donations to charitable causes and intends to continue to contribute and eventually fulfill her pledge."

She adds ... "However, Amber has been delayed in that goal because Mr. Depp filed a lawsuit against her, and consequently, she has been forced to spend millions of dollars defending Mr. Depp's false accusations against her."