Although Yitschaq would assume Yaaqov deceived him, [Bereshit 27:12-13] established Yaaqov as innocent & Rivqah as the deceiver.
Yitschaq was innocently unaware of Rivqah's plan, when witnessing against Yaaqov in [Bereshit 27:35]. - The result of Yitschaq's testimony to Esav could potentially represent 'lashon hara' as described in [Shemot 23:7]. - Yitschaq's testimony caused Esav in [Bereshit 27:41] to vow harm upon Yaaqov (whose innocence was unknowingly declared by Rivqah).
What Halakhic consequences apply to Yitschaq if found guilty of Lashon Hara?
Chofetz Chaim - [ The Prohibition Against Lashon Hara , Principle 4 : 12 ]
And if he transgressed and spoke lashon hara about his friend and came to repent, it [his repentance] depends upon this: If his friends rejected his words and his friend was in no way demeaned by this [lashon hara] in their eyes, if so, there adheres to him only the sin of "between man and his Maker" (and not that of between "man and his neighbor"), his having transgressed the will of the L–rd, who commanded this [that lashon hara must not be spoken], as we wrote above in the introduction. His correction is to regret what has passed, confess [his sin] and take it upon himself with a full heart not to repeat this [sin] in the future, as with all sins between man and his Maker.
But if his friend were demeaned by this in the eyes of the hearers and through this suffered physical or financial harm, or if he were caused [emotional] pain by this, this is in the category of all the sins between man and his neighbor, which even Yom Kippur and the day of death do not atone for until he conciliates his neighbor. He must, therefore, ask pardon of his friend for this, and when he is conciliated and forgives him, there remains with him only the sin of between man and his Maker, and he must do as mentioned above.
And even if his friend does not yet know anything about it, he must reveal what he did to him which was not in accordance with the din, and ask forgiveness of him for this, since he knows that through him this harm was done him. From this we can understand how much one must take care to guard himself from this pernicious trait [of lashon hara], for if one is steeped in this, G–d forbid, teshuvah [repentance] is almost impossible for him. For he certainly will not remember all of the souls whom he grieved by this lashon hara. And even those people whom he remembers as having stirred up evil against will not know of it, wherefore he will be ashamed to reveal it to them. And sometimes 54 he will speak of a family taint and thereby harm all the future generations, so that he can never be pardoned for this.
As Chazal have said (Yerushalmi Bava Kamma 8:7): "One who speaks of a family taint never has atonement [for this]." Therefore, one must distance himself from this extremely pernicious trait, so that he not thereafter be, G–d forbid, [in the category of] "the crooked cannot be straightened" (Koheleth 1:15).